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Why Read This Book? - Index of

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154 Chapter 4 Functions<br />

Example 4.9.6 Suppose you toss a coin 10 times and observe the sequence <strong>of</strong><br />

outcomes. From Example 4.8.14, we know that there are 2 10 possible outcomes.<br />

We want to count the number <strong>of</strong> these outcomes that have exactly 3 heads. Imagine<br />

we have the numbers in N10 ={1,...,10} written on slips <strong>of</strong> paper. An outcome<br />

<strong>of</strong> the 10 coin tosses with exactly 3 heads can be uniquely specified by taking a<br />

3-element subset <strong>of</strong> N10 to represent which <strong>of</strong> the 10 slots are heads. There are<br />

C(10, 3) = (10 · 9 · 8)/(3 · 2 · 1) = 120 such choices. �<br />

Example 4.9.7 A box <strong>of</strong> light bulbs contains 100 bulbs, 3 <strong>of</strong> which are defective.<br />

If we choose 10 bulbs from the box, we want to calculate how many ways there<br />

are to get precisely one <strong>of</strong> the defective bulbs. Such a choice <strong>of</strong> 10 bulbs consists<br />

<strong>of</strong> 9 <strong>of</strong> the 97 good bulbs chosen without regard to order, and 1 <strong>of</strong> the defective<br />

bulbs chosen from the 3 bulbs. Multiplying these, we have C(97, 9) × C(3, 1) ways<br />

<strong>of</strong> choosing the bulbs in which precisely one is defective. �<br />

Exercise 4.6.7 says that if A and B are disjoint, then |A ∪ B| = |A| + |B|. Applied<br />

to counting the number <strong>of</strong> ways <strong>of</strong> performing a task, we call this theorem the<br />

sum rule. If counting the number <strong>of</strong> ways <strong>of</strong> performing a task must be broken up<br />

into disjoint cases, the sum rule says simply that the calculations for the different<br />

cases are added to produce the total number <strong>of</strong> ways <strong>of</strong> performing the task.<br />

Example 4.9.8 A license plate consists <strong>of</strong> up to seven characters, taken from<br />

the 26 letters and the digits 0–9. If there is room for spaces, they are not counted<br />

in the arrangement; for example, there is no distinction between TANGENT<br />

and TAN GENT . We count the total number <strong>of</strong> plates that can be issued. There<br />

are different cases based on the number <strong>of</strong> characters used. The number <strong>of</strong> onecharacter<br />

license plates is 36; the number <strong>of</strong> two-character license plates is 362 ,<br />

and so on. Thus the total number <strong>of</strong> license plates is<br />

36 + 36 2 + 36 3 +···+36 7 = 36(1 +···+36 6 ) = 36 � 367 � −1 36<br />

36−1 = 35 (367 − 1) �<br />

Example 4.9.9 Your club <strong>of</strong> 30 women and 20 men is going to choose a committee<br />

<strong>of</strong> five. We calculate the number <strong>of</strong> ways to choose the committee if it must<br />

include at least one man. The number <strong>of</strong> ways to choose the committee with no<br />

restrictions is C(50, 5), and the number <strong>of</strong> ways to choose the committee with<br />

no men is C(30, 5). By the complement rule, the number <strong>of</strong> ways to choose the<br />

committee where there is at least one man is C(50, 5) − C(30, 5). �<br />

EXERCISE 4.9.10 Pizza Peddler <strong>of</strong>fers 12 different toppings and is having a<br />

special on their two-topping pizzas. How many distinct ways are there to order<br />

one <strong>of</strong> their special pizzas?<br />

EXERCISE 4.9.11 You overheard that a TRUE/FALSE test <strong>of</strong> 10 questions<br />

had 7 TRUE and 3 FALSE answers. If you take the test with this knowledge, how<br />

many ways are there to fill out the answer sheet?

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