Thinking Mathematically (6th Edition)

Published by Pearson
ISBN 10: 0321867327
ISBN 13: 978-0-32186-732-2

Chapter 11 - Counting Methods and Probability Theory - 11.3 Combinations - Exercise Set 11.3: 19

Answer

7

Work Step by Step

${}_{n}C_{r}=\displaystyle \frac{n!}{(n-r)!r!}$. ----------------- ${}_{7}C_{3}=\displaystyle \frac{7!}{(7-3)!3!}=\frac{7\times 6\times 5}{1\times 2\times 3}=35$ ${}_{5}C_{4}=\displaystyle \frac{5!}{(5-4)!4!}=\frac{5}{1}=5$ $\displaystyle \frac{ {}_{7}C_{3}}{ {}_{5}C_{4}}=\frac{35}{5}=7$
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