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  • Chapter 11Sets and Counting

    Chapter Overview

    In this chapter, you will learn to:

    1. Use set theory and Venn diagrams to solve counting problems.

    2. Use the Multiplication Axiom to solve counting problems.

    3. Use Permutations to solve counting problems.

    4. Use Combinations to solve counting problems.

    5. Use the Binomial Theorem to expand (x+y)n.

    Sets

    In this section, we will familiarize ourselves with set operations and notations, so that we can apply these concepts to both counting and probability problems. We begin by defining some terms.

    A set is a collection of objects, and its members are called the elements of the set. We name the set by using capital letters, and enclose its members in braces. Suppose we need to list the members of the chess club. We use the following set notation.

    (11.1) C = {Ken, Bob, Tran, Shanti, Eric}

    A set that has no members is called an empty set. The empty set is denoted by the symbol Ø.

    Two sets are equal if they have the same elements.

    A set A is a subset of a set B if every member of A is also a member of B.

    Suppose C={Al, Bob, Chris, David, Ed} and A={Bob, David}. Then A is a subset of C, written as AC.

    Every set is a subset of itself, and the empty set is a subset of every set.

    Union Of Two Sets

    Let A and B be two sets, then the union of A and B, written as AB, is the set of all elements that are either in A or in B, or in both A and B.

    Intersection Of Two Sets

    Let A and B be two sets, then the intersection of A and B, written as AB, is the set of all elements that are common to both sets A and B.

    A universal set U is the set consisting of all elements under consideration.

    Complement of a Set

    Let A be any set, then the complement of set A, written as _autogen-svg2png-0031.png, is the set consisting of elements in the universal set U that are not in A.

    Disjoint Sets

    Two sets A and B are called disjoint sets if their intersection is an empty set.

    Example 11.1

    List all the subsets of the set of primary colors {red, yellow, blue}.

    The subsets are ∅, {red}, {yellow}, {blue}, {red, yellow}, {red, blue}, {yellow, blue}, {red, yellow, blue}

    Note that the empty set is a subset of every set, and a set is a subset of itself.

    Example 11.2

    Let F={Aikman, Jackson, Rice, Sanders, Young}, and B={Griffey, Jackson, Sanders, Thomas}. Find the intersection of the sets F and B.

    The intersection of the two sets is the set whose elements belong to both sets. Therefore,

    (11.2)FB={Jackson, Sanders}
    Example 11.3

    Find the union of the sets F and B given as follows.

    ()F={Aikman, Jackson, Rice, Sanders, Young}B={Griffey, Jackson, Sanders, Thomas}

    The union of two sets is the set whose elements are either in A or in B or in both A and B. Therefore

    (11.3)FB={Aikman, Griffey, Jackson, Rice, Sanders, Thomas, Young}

    Observe that when writing the union of two sets, the repetitions are avoided.

    Example 11.4

    Let the universal set U={red, orange, yellow, green, blue, indigo, violet}, and P={red, yellow, blue}. Find the complement of P.

    The complement of a set P is the set consisting of elements in the universal set U that are not in P. Therefore,

    (11.4)
    _autogen-svg2png-0063.png

    To achieve a better understanding, let us suppose that the universal set U represents the colors of the spectrum, and P the primary colors, then _autogen-svg2png-0066.png represents those colors of the spectrum that are not primary colors.

    Example 11.5

    Let U={red, orange, yellow, green, blue, indigo, violet}, P={red, yellow, blue}, Q={red, green}, and R={orange, green, indigo}. Find _autogen-svg2png-0071.png.

    We do the problems in steps.

    (11.5)
    _autogen-svg2png-0072.png

    We now use Venn diagrams to illustrate the relations between sets. In the late 1800s, an English logician named John Venn developed a method to represent relationship between sets. He represented these relationships using diagrams, which are now known as Venn diagrams. A Venn diagram represents a set as the interior of a circle. Often two or more circles are enclosed in a rectangle where the rectangle represents the universal set. To visualize an intersection or union of a set is easy. In this section, we will mainly use Venn diagrams to sort various populations and count objects.

    Example 11.6

    Suppose a survey of car enthusiasts showed that over a certain time period, 30 drove cars with automatic transmissions, 20 drove cars with standard transmissions, and 12 drove cars of both types. If every one in the survey drove cars with one of these transmissions, how many people participated in the survey?

    We will use Venn diagrams to solve this problem.

    Let the set A represent those car enthusiasts who drove cars with automatic transmissions, and set S represent the car enthusiasts who drove the cars with standard transmissions. Now we use Venn diagrams to sort out the information given in this problem.

    Since 12 people drove both cars, we place the number 12 in the region common to both sets.

    replace1.png
    (a)
    replace2.png
    (b)
    replace3.png
    (c)
    Figure 11.0

    Because 30 people drove cars with automatic transmissions, the circle A must contain 30 elements. This means x+12=30, or x=18. Similarly, since 20 people drove cars with standard transmissions, the circle B must contain 20 elements, or y+12=20 which in turn makes y=8.

    Now that all the information is sorted out, it is easy to read from the diagram that 18 people drove cars with automatic transmissions only, 12 people drove both types of cars, and 8 drove cars with standard transmissions only. Therefore, 18+12+8=38 people took part in the survey.

    Example 11.7

    A survey of 100 people in California indicates that 60 people have visited Disneyland, 15 have visited Knott's Berry Farm, and 6 have visited both. How many people have visited neither place?

    The problem is similar to the one in Example 11.6.

    Let the set D represent the people who have visited Disneyland, and K the set of people who have visited Knott's Berry Farm.

    replace6_3.png
    (a)
    replace6_4.png
    (b)
    Figure 11.0

    We fill the three regions associated with the sets D and K in the same manner as before. Since 100 people participated in the survey, the rectangle representing the universal set U must contain 100 objects. Let x represent those people in the universal set that are neither in the set D nor in K. This means 54+6+9+x=100, or x=31.

    Therefore, there are 31 people in the survey who have visited neither place.

    Example 11.8

    A survey of 100 exercise conscious people resulted in the following information:

    • 50 jog, 30 swim, and 35 cycle

    • 14 jog and swim

    • 7 swim and cycle

    • 9 jog and cycle

    • 3 people take part in all three activities

    1. How many jog but do not swim or cycle?

    2. How many take part in only one of the activities?

    3. How many do not take part in any of these activities?

    Let J represent the set of people who jog, S the set of people who swim, and C who cycle.

    In using Venn diagrams, our ultimate aim is to assign a number to each region. We always begin by first assigning the number to the innermost region and then working our way out.

    replace6_5.png
    (a)
    replace6_6.png
    (b)
    replace6_7.png
    (c)
    Figure 11.0

    We place a 3 in the innermost region of Figure 11.0a because it represents the number of people who participate in all three activities. Next we compute x, y and z.

    Since 14 people jog and swim, x+3=14, or x=11.

    The fact that 9 people jog and cycle results in y+3=9, or y=6.

    Since 7 people swim and cycle, z+3=7, or z=4.

    This information is depicted in Figure 11.0b.

    Now we proceed to find the unknowns m, n and p.

    Since 50 people jog, m+11+6+3=50, or m=30.

    Thirty people swim, therefore, n+11+4+3=30, or n=12.

    Thirty five people cycle, therefore, p+6+4+3=35, or p=22.

    By adding all the entries in all three sets, we get a sum of 88. Since 100 people were surveyed, the number inside the universal set but outside of all three sets is 100 – 88, or 12.

    In Figure 11.0c, the information is sorted out, and the questions can readily be answered.

    1. The number of people who jog but do not swim or cycle is 30.

    2. The number who take part in only one of these activities is 30+12+22=64.

    3. The number of people who do not take part in any of these activities is 12.

    Tree Diagrams and the Multiplication Axiom

    In this chapter, we are trying to develop counting techniques that will be used in the Section 13.1 to study probability. One of the most fundamental of such techniques is called the Multiplication Axiom. Before we introduce the multiplication axiom, we first look at some examples.

    Example 11.9

    If a woman has two blouses and three skirts, how many different outfits consisting of a blouse and a skirt can she wear?

    Suppose we call the blouses b1 and  b2,  and skirts  s1,  s2,  and   s3.

    We can have the following six outfits.

    (11.6)b1s1,b1s2,b1s3,b2s1,b2s2,b2s3

    Alternatively, we can draw a tree diagram:

    graphics4.png
    Figure 11.0

    The tree diagram gives us all six possibilities. The method involves two steps. First the woman chooses a blouse. She has two choices: blouse one or blouse two. If she chooses blouse one, she has three skirts to match it with; skirt one, skirt two, or skirt three. Similarly if she chooses blouse two, she can match it with each of the three skirts, again. The tree diagram helps us visualize these possibilities.

    The reader should note that the process involves two steps. For the first step of choosing a blouse, there are two choices, and for each choice of a blouse, there are three choices of choosing a skirt. So altogether there are 2⋅3=6 possibilities.

    If, in the above example, we add the shoes to the outfit, we have the following problem.

    Example 11.10

    If a woman has two blouses, three skirts, and two pumps, how many different outfits consisting of a blouse, a skirt, and a pair of pumps can she wear?

    Suppose we call the blouses b1 and b2, the skirts s1, s2, and s3, and the pumps p1, and p2.

    The following tree diagram results.

    graphics5.png
    Figure 11.0

    We count the number of branches in the tree, and see that there are 12 different possibilities. This time the method involves three steps. First, the woman chooses a blouse. She has two choices: blouse one or blouse two. Now suppose she chooses blouse one. This takes us to step two of the process which consists of choosing a skirt. She has three choices for a skirt, and let us suppose she chooses skirt two. Now that she has chosen a blouse and a skirt, we have moved to the third step of choosing a pair of pumps. Since she has two pairs of pumps, she has two choices for the last step. Let us suppose she chooses pumps two. She has chosen the outfit consisting of blouse one, skirt two, and pumps two, or b1s2p2. By looking at the different branches on the tree, one can easily see the other possibilities.

    The important thing to observe here, again, is that this is a three step process. There are two choices for the first step of choosing a blouse. For each choice of a blouse, there are three choices of choosing a skirt, and for each combination of a blouse and a skirt, there are two choices of selecting a pair of pumps. All in all, we have 2⋅3⋅2=12 different possibilities.

    The tree diagrams help us visualize the different possibilities, but they are not practical when the possibilities are numerous. Besides, we are mostly interested in finding the number of elements in the set and not the actual possibilities. But once the problem is envisioned, we can solve it without a tree diagram. The two examples we just solved may have given us a clue to do just that.

    Let us now try to solve Example 11.10 without a tree diagram. Recall that the problem involved three steps: choosing a blouse, choosing a skirt, and choosing a pair of pumps. The number of ways of choosing each are listed below.

    Table 11.1.
    The Number of ways of choosing a blouseThe number of ways of choosing a skirtThe number of ways of choosing pumps
    232

    By multiplying these three numbers we get 12, which is what we got when we did the problem using a tree diagram.

    The procedure we just employed is called the multiplication axiom.

    Example 11.11THE MULTIPLICATION AXIOM

    If a task can be done in m ways, and a second task can be done in n ways, then the operation involving the first task followed by the second can be performed in m· n ways.

    The general multiplication axiom is not limited to just two tasks and can be used for any number of tasks.

    Example 11.12

    A truck license plate consists of a letter followed by four digits. How many such license plates are possible?

    Since there are 26 letters and 10 digits, we have the following choices for each.

    Table 11.2.
    LetterDigitDigitDigitDigit
    2610101010

    Therefore, the number of possible license plates is 26⋅10⋅10⋅10⋅10=260,000.

    Example 11.13

    In how many different ways can a 3-question true-false test be answered?

    Since there are two choices for each question, we have

    Table 11.3.
    Question 1Question 2Question 3
    222

    Applying the multiplication axiom, we get 2⋅2⋅2=8 different ways.

    We list all eight possibilities below.

    TTT, TTF, TFT, TFF, FTT, FTF, FFT, FFF

    The reader should note that the first letter in each possibility is the answer corresponding to the first question, the second letter corresponds to the answer to the second question and so on. For example, TFF, says that the answer to the first question is given as true, and the answers to the second and third questions false.

    Example 11.14

    In how many different ways can four people be seated in a row?

    Suppose we put four chairs in a row, and proceed to put four people in these seats.

    There are four choices for the first chair we choose. Once a person sits down in that chair, there are only three choices for the second chair, and so on. We list as shown below.

    Table 11.4.
    4321

    So there are altogether 4⋅3⋅2⋅1=24 different ways.

    Example 11.15

    How many three-letter word sequences can be formed using the letters {A,B,C} if no letter is to be repeated?

    The problem is very similar to Example 11.14.

    Imagine a child having three building blocks labeled A, B, and C. Suppose he puts these blocks on top of each other to make word sequences. For the first letter he has three choices, namely A, B, or C. Let us suppose he chooses the first letter to be a B, then for the second block which must go on top of the first, he has only two choices: A or C. And for the last letter he has only one choice. We list the choices below.

    Table 11.5.
    321

    Therefore, 6 different word sequences can be formed.

    Finally, we'd like to illustrate this with a tree diagram.

    graphics6.png
    Figure 11.0

    All six possibilities are displayed in the tree diagram.

    Permutations

    In Example 11.15, we were asked to find the word sequences formed by using the letters {A,B,C} if no letter is to be repeated. The tree diagram gave us the following six arrangements.

    _autogen-svg2png-0127.png, _autogen-svg2png-0128.png, _autogen-svg2png-0129.png, _autogen-svg2png-0130.png, _autogen-svg2png-0131.png, and _autogen-svg2png-0132.png,

    Arrangements like these, where order is important and no element is repeated, are called permutations.

    Example 11.16Permutations

    A permutation of a set of elements is an ordered arrangement where each element is used once.

    Example 11.17

    How many three-letter word sequences can be formed using the letters {A,B,C,D}?

    There are four choices for the first letter of our word, three choices for the second letter, and two choices for the third.

    Table 11.6.
    432

    Applying the multiplication axiom, we get 4⋅3⋅2=24 different arrangements.

    Example 11.18

    How many permutations of the letters of the word ARTICLE have consonants in the first and last positions?

    In the word ARTICLE, there are 4 consonants.

    Since the first letter must be a consonant, we have four choices for the first position, and once we use up a consonant, there are only three consonants left for the last spot. We show as follows:

    Table 11.7.
    4     3

    Since there are no more restrictions, we can go ahead and make the choices for the rest of the positions.

    So far we have used up 2 letters, therefore, five remain. So for the next position there are five choices, for the position after that there are four choices, and so on. We get

    Table 11.8.
    4543213

    So the total permutations are 4⋅5⋅4⋅3⋅2⋅1⋅3=1440.

    Example 11.19

    Given five letters {A,B,C,D,E}. Find the following:

    1. The number of four-letter word sequences.

    2. The number of three-letter word sequences.

    3. The number of two-letter word sequences.

    The problem is easily solved by the multiplication axiom, and answers are as follows:

    1. a. The number of four-letter word sequences is 5⋅4⋅3⋅2=120.

    2. b. The number of three-letter word sequences is 5⋅4⋅3=60.

    3. c. The number of two-letter word sequences is 5⋅4=20.

    We often encounter situations where we have a set of n objects and we are selecting r objects to form permutations. We refer to this as permutations of n objects taken r at a time, and we write it as nPr.

    Therefore, Example 11.19 can also be answered as listed below.

    1. The number of four-letter word sequences is 5P4=120.

    2. The number of three-letter word sequences is 5P3=60.

    3. The number of two-letter word sequences is 5P2=20.

    Before we give a formula for nPr, we'd like to introduce a symbol that we will use a great deal in this as well as in Section 13.1.

    Example 11.20Factorial

    n!=n(n−1)(n−2)(n−3)⋯3⋅2⋅1.

    Where n is a natural number.

    (11.7) 0 ! = 1

    Now we define nPr.

    Example 11.21 The Number of Permutations of n Objects Taken r at a Time

    nPr=n(n−1)(n−2)(n−3)⋯(nr+1), or
    _autogen-svg2png-0156.png

    Where n and r are natural numbers.

    The reader should become familiar with both formulas and should feel comfortable in applying either.

    Example 11.22

    Compute the following using both formulas.

    1. 6P3

    2. 7P2

    We will identify n and r in each case and solve using the formulas provided.

    1. 6P3=6⋅5⋅4=120, alternately _autogen-svg2png-0164.png

    2. 7P2=7⋅6=42, or _autogen-svg2png-0166.png

    Next we consider some more permutation problems to get further insight into these concepts.

    Example 11.23

    In how many different ways can 4 people be seated in a straight line if two of them insist on sitting next to each other?

    Let us suppose we have four people A, B, C, and D. Further suppose that A and B want to sit together. For the sake of argument, we tie A and B together and treat them as one person.

    The four people are _autogen-svg2png-0175.png_autogen-svg2png-0176.png. Since _autogen-svg2png-0177.png is treated as one person, we have the following possible arrangements.

    (11.8)
    _autogen-svg2png-0178.png

    Note that there are six more such permutations because A and B could also be tied in the order _autogen-svg2png-0181.png. And they are

    (11.9)
    _autogen-svg2png-0182.png

    So altogether there are 12 different permutations.

    Let us now do the problem using the multiplication axiom.

    After we tie two of the people together and treat them as one person, we can say we have only three people. The multiplication axiom tells us that three people can be seated in 3! ways. Since two people can be tied together 2! ways, there are 3!2!=12 different arrangements.

    Example 11.24

    You have 4 math books and 5 history books to put on a shelf that has 5 slots. In how many ways can the books be shelved if the first three slots are filled with math books and the next two slots are filled with history books?

    We first do the problem using the multiplication axiom.

    Since the math books go in the first three slots, there are 4 choices for the first slot, 3 for the second and 2 for the third. The fourth slot requires a history book, and has five choices. Once that choice is made, there are 4 history books left, and therefore, 4 choices for the last slot. The choices are shown below.

    Table 11.9.
    43254

    Therefore, the number of permutations are 4⋅3⋅2⋅5⋅4=480.

    Alternately, we can see that 4⋅3⋅2 is really same as 4P3, and 5⋅4 is 5P2.

    So the answer can be written as _autogen-svg2png-0191.png.

    Clearly, this makes sense. For every permutation of three math books placed in the first three slots, there are 5P2 permutations of history books that can be placed in the last two slots. Hence the multiplication axiom applies, and we have the answer _autogen-svg2png-0193.png.

    We summarize.

    1. Permutations

      A permutation of a set of elements is an ordered arrangement where each element is used once.

    2. Factorial

      n!=n(n−1)(n−2)(n−3)⋯3⋅2⋅1.

      Where n is a natural number.

      (11.10) 0 ! = 1
    3. Permutations of n Objects Taken r at a Time

      nPr=n(n−1)(n−2)(n−3)⋯(nr+1), or _autogen-svg2png-0200.png

      Where n and r are natural numbers.

    Circular Permutations and Permutations with Similar Elements

    Section Overview

    In this section we will address the following two problems.

    1. In how many different ways can five people be seated in a circle?

    2. In how many different ways can the letters of the word MISSISSIPPI be arranged?

    The first problem comes under the category of Circular Permutations, and the second under Permutations with Similar Elements.

    Circular Permutations

    Suppose we have three people named A, B, and C. We have already determined that they can be seated in a straight line in 3! or 6 ways. Our next problem is to see how many ways these people can be seated in a circle. We draw a diagram.

    graphics7.png
    Figure 11.1

    It happens that there are only two ways we can seat three people in a circle. This kind of permutation is called a circular permutation. In such cases, no matter where the first person sits, the permutation is not affected. Each person can shift as many places as they like, and the permutation will not be changed. Imagine the people on a merry-go-round; the rotation of the permutation does not generate a new permutation. So in circular permutations, the first person is considered a place holder, and where he sits does not matter.

    Example 11.25Circular Permutations

    The number of permutations of n elements in a circle is (n−1)!

    Example 11.26

    In how many different ways can five people be seated at a circular table?

    We have already determined that the first person is just a place holder. Therefore, there is only one choice for the first spot. We have

    Table 11.10.
    14321

    So the answer is 24.

    Example 11.27

    In how many ways can four couples be seated at a round table if the men and women want to sit alternately?

    We again emphasize that the first person can sit anywhere without affecting the permutation.

    So there is only one choice for the first spot. Suppose a man sat down first. The chair next to it must belong to a woman, and there are 4 choices. The next chair belongs to a man, so there are three choices and so on. We list the choices below.

    Table 11.11.
    14332211

    So the answer is 144.

    Now we address the second problem.

    Permutations with Similar Elements

    Let us determine the number of distinguishable permutations of the letters ELEMENT.

    Suppose we make all the letters different by labeling the letters as follows.

    (11.11)
    _autogen-svg2png-0209.png

    Since all the letters are now different, there are 7! different permutations.

    Let us now look at one such permutation, say

    (11.12)
    _autogen-svg2png-0211.png

    Suppose we form new permutations from this arrangement by only moving the E's. Clearly, there are 3! or 6 such arrangements. We list them below.

    (11.13)
    _autogen-svg2png-0213.png
    (11.14)
    _autogen-svg2png-0214.png
    (11.15)
    _autogen-svg2png-0215.png
    (11.16)
    _autogen-svg2png-0216.png
    (11.17)
    _autogen-svg2png-0217.png
    (11.18)
    _autogen-svg2png-0218.png

    Because the E's are not different, there is only one arrangement _autogen-svg2png-0220.png and not six. This is true for every permutation.

    Let us suppose there are n different permutations of the letters _autogen-svg2png-0222.png.

    Then there are n⋅3! permutations of the letters _autogen-svg2png-0224.png.

    But we know there are 7! permutations of the letters _autogen-svg2png-0226.png.

    Therefore, n⋅3!=7!

    Or _autogen-svg2png-0228.png.

    This gives us the method we are looking for.

    Example 11.28Permutations with Similar Elements

    The number of permutations of n elements taken n at a time, with r1 elements of one kind, r2 elements of another kind, and so on, is

    (11.19)
    _autogen-svg2png-0233.png
    Example 11.29

    Find the number of different permutations of the letters of the word MISSISSIPPI.

    The word MISSISSIPPI has 11 letters. If the letters were all different there would have been 11! different permutations. But MISSISSIPPI has 4 S's, 4 I's, and 2 P's that are alike.

    So the answer is _autogen-svg2png-0235.png

    Which equals 34,650.

    Example 11.30

    If a coin is tossed six times, how many different outcomes consisting of 4 heads and 2 tails are there?

    Again, we have permutations with similar elements.

    We are looking for permutations for the letters HHHHTT.

    The answer is _autogen-svg2png-0236.png.

    Example 11.31

    In how many different ways can 4 nickels, 3 dimes, and 2 quarters be arranged in a row?

    Assuming that all nickels are similar, all dimes are similar, and all quarters are similar, we have permutations with similar elements. Therefore, the answer is

    (11.20)
    _autogen-svg2png-0237.png
    Example 11.32

    A stock broker wants to assign 20 new clients equally to 4 of its salespeople. In how many different ways can this be done?

    This means that each sales person gets 5 clients. The problem can be thought of as an ordered partitions problem. In that case, using the formula we get

    (11.21)
    _autogen-svg2png-0238.png

    We summarize.

    Example 11.33
    1. Circular Permutations

      The number of permutations of n elements in a circle is

      (11.22) ( n − 1 ) !
    2. Permutations with Similar Elements

      The number of permutations of n elements taken n at a time, with r1 elements of one kind, r2 elements of another kind, and so on, such that n=r1+r2+⋯+rk is

      _autogen-svg2png-0246.png

      This is also referred to as ordered partitions.

    Combinations

    Suppose we have a set of three letters {A,B,C}, and we are asked to make two-letter word sequences. We have the following six permutations.

    _autogen-svg2png-0248.png _autogen-svg2png-0249.png _autogen-svg2png-0250.png _autogen-svg2png-0251.png _autogen-svg2png-0252.png _autogen-svg2png-0253.png

    Now suppose we have a group of three people {A,B,C} as Al, Bob, and Chris, respectively, and we are asked to form committees of two people each. This time we have only three committees, namely,

    _autogen-svg2png-0255.png _autogen-svg2png-0256.png _autogen-svg2png-0257.png

    When forming committees, the order is not important, because the committee that has Al and Bob is no different than the committee that has Bob and Al. As a result, we have only three committees and not six.

    Forming word sequences is an example of permutations, while forming committees is an example of combinations – the topic of this section.

    Permutations are those arrangements where order is important, while combinations are those arrangements where order is not significant. From now on, this is how we will tell permutations and combinations apart.

    In Example 11.32, there were six permutations, but only three combinations.

    Just as the symbol nPr represents the number of permutations of n objects taken r at a time, _autogen-svg2png-0261.png represents the number of combinations of n objects taken r at a time.

    So in Example 11.32, 3P2=6, and 3C2=3.

    Our next goal is to determine the relationship between the number of combinations and the number of permutations in a given situation.

    In Example 11.32, if we knew that there were three combinations, we could have found the number of permutations by multiplying this number by 2!. That is because each combination consists of two letters, and that makes 2! permutations.

    Example 11.34

    Given the set of letters {A,B,C,D}. Write the number of combinations of three letters, and then from these combinations determine the number of permutations.

    We have the following four combinations.

    ABC BCD CDA BDA

    Since every combination has three letters, there are 3! permutations for every combination. We list them below.

    ABC BCD CDA BDA

    ACB BDC CAD BAD

    BAC CDB DAC DAB

    BCA CBD DCA DBA

    CAB DCB ACD ADB

    CBA DBC ADC ABD

    The number of permutations are 3! times the number of combinations. That is

    (11.23)4p3=3!⋅4C3

    or _autogen-svg2png-0272.png

    In general, _autogen-svg2png-0273.png

    Since _autogen-svg2png-0274.png

    We have, _autogen-svg2png-0275.png

    Summarizing,

    Example 11.35
    1. Combinations

      A combination of a set of elements is an arrangement where each element is used once, and order is not important.

    2. The Number of Combinations of n Objects Taken r at a Time

      _autogen-svg2png-0278.png

      Where n and r are natural numbers.

    Example 11.36

    Compute:

    1. 5C3

    2. 7C3.

    We use the above formula.

    (11.24)
    _autogen-svg2png-0283.png
    (11.25)
    _autogen-svg2png-0284.png
    Example 11.37

    In how many different ways can a student select to answer five questions from a test that has seven questions, if the order of the selection is not important?

    Since the order is not important, it is a combination problem, and the answer is

    7C5=21.

    Example 11.38

    How many line segments can be drawn by connecting any two of the six points that lie on the circumference of a circle?

    Since the line that goes from point A to point B is same as the one that goes from B to A, this is a combination problem.

    It is a combination of 6 objects taken 2 at a time. Therefore, the answer is

    (11.26)
    _autogen-svg2png-0290.png
    Example 11.39

    There are ten people at a party. If they all shake hands, how many hand-shakes are possible?

    Note that between any two people there is only one hand shake. Therefore, we have

    (11.27)10C2=45   hand-shakes.
    Example 11.40

    The shopping area of a town is in the shape of square that is 5 blocks by 5 blocks. How many different routes can a taxi driver take to go from one corner of the shopping area to the opposite cater-corner?

    Let us suppose the taxi driver drives from the point A, the lower left hand corner, to the point B, the upper right hand corner as shown in the figure below.

    graphics8.png
    Figure 11.1

    To reach his destination, he has to travel ten blocks; five horizontal, and five vertical. So if out of the ten blocks he chooses any five horizontal, the other five will have to be the vertical blocks, and vice versa. Therefore, all he has to do is to choose 5 out of ten.

    The answer is 10C5, or 252.

    Alternately, the problem can be solved by permutations with similar elements.

    The taxi driver's route consists of five horizontal and five vertical blocks. If we call a horizontal block H, and a vertical block a V, then one possible route may be as follows.

    (11.28)
    _autogen-svg2png-0296.png

    Clearly there are _autogen-svg2png-0297.png.

    Further note that by definition _autogen-svg2png-0298.png.

    Example 11.41

    If a coin is tossed six times, in how many ways can it fall four heads and two tails?

    First we solve this problem using the section called “Combinations” technique–permutations with similar elements.

    We need 4 heads and 2 tails, that is

    (11.29)
    _autogen-svg2png-0299.png

    There are _autogen-svg2png-0300.png permutations.

    Now we solve this problem using combinations.

    Suppose we have six spots to put the coins on. If we choose any four spots for heads, the other two will automatically be tails. So the problem is simply

    6C4=15.

    Incidentally, we could have easily chosen the two tails, instead. In that case, we would have gotten

    6C2=15.

    Further observe that by definition

    (11.30)
    _autogen-svg2png-0303.png

    and _autogen-svg2png-0304.png

    Which implies

    (11.31)6C4=6C2.

    Combinations: Involving Several Sets

    So far we have solved the basic combination problem of r objects chosen from n different objects. Now we will consider certain variations of this problem.

    Example 11.42

    How many five-people committees consisting of 2 men and 3 women can be chosen from a total of 4 men and 4 women?

    We list 4 men and 4 women as follows:

    (11.32)M1M2M3M4W1W2W3W4

    Since we want 5-people committees consisting of 2 men and 3 women, we'll first form all possible two-man committees and all possible three-woman committees. Clearly there are 4C2 = 6 two-man committees, and 4C3 = 4 three-woman committees, we list them as follows:

    Table 11.12.
    2-Man Committees3-Woman Committees
    M 1 M 2 W 1 W 2 W 3
    M 1 M 3 W 1 W 2 W 4
    M 1 M 4 W 1 W 3 W 4
    M 2 M 3 W 2 W 3 W 4
    M 2 M 4  
    M 3 M 4  

    For every 2-man committee there are four 3-woman committees that can be chosen to make a 5-person committee. If we choose M1M2 as our 2-man committee, then we can choose any of W1W2W3, W1W2W4, W1W3W4, or W2W3W4 as our 3-woman committees. As a result, we get

    (11.33)
    _autogen-svg2png-0324.png

    Similarly, if we choose M1M3 as our 2-man committee, then, again, we can choose any of W1W2W3, W1W2W4, W1W3W4, or W2W3W4 as our 3-woman committees.

    (11.34)
    _autogen-svg2png-0330.png

    And so on.

    Since there are six 2-man committees, and for every 2-man committee there are four 3- woman committees, there are altogether 6⋅4=24 five-people committees.

    In essence, we are applying the multiplication axiom to the different combinations.

    Example 11.43

    A high school club consists of 4 freshmen, 5 sophomores, 5 juniors, and 6 seniors. How many ways can a committee of 4 people be chosen that includes

    1. One student from each class?

    2. All juniors?

    3. Two freshmen and 2 seniors?

    4. No freshmen?

    5. At least three seniors?

    1. Applying the multiplication axiom to the combinations involved, we get

      (11.35)4C1⋅5C1⋅5C1⋅6C1=600
    2. We are choosing all 4 members from the 5 juniors, and none from the others.

      (11.36)5C4=5
    3. 4C2⋅6C2=90

    4. Since we don't want any freshmen on the committee, we need to choose all members from the remaining 16. That is

      (11.37)16C4=1820
    5. Of the 4 people on the committee, we want at least three seniors. This can be done in two ways. We could have three seniors, and one non-senior, or all four seniors.

      (11.38)6C3⋅14C1+6C4=295
    Example 11.44

    How many five-letter word sequences consisting of 2 vowels and 3 consonants can be formed from the letters of the word INTRODUCE?

    First we select a group of five letters consisting of 2 vowels and 3 consonants. Since there are 4 vowels and 5 consonants, we have

    (11.39)4C2⋅5C3

    Since our next task is to make word sequences out of these letters, we multiply these by 5!.

    4C2⋅5C3⋅5!=7200.

    Example 11.45

    A standard deck of playing cards has 52 cards consisting of 4 suits each with 13 cards. In how many different ways can a 5-card hand consisting of four cards of one suit and one of another be drawn?

    We will do the problem using the following steps. Step 1. Select a suit. Step 2. Select four cards from this suit. Step 3. Select another suit. Step 4. Select a card from that suit.

    Applying the multiplication axiom, we have

    Table 11.13.
    Ways of selecting a suitWays if selecting 4 cards from this suitWays if selecting the next suitWays of selecting a card from that suit
    4C1 13 C4 3C1 13 C1
    (11.40)4C1⋅13C4⋅3C1⋅13C1=111,540.
    Solutions