Chapter 7 - Section 7.6 - Counting Theory - 7.6 Exercises - Page 679: 21

$1,860,480$

Using $P(n,r)=\dfrac{n!}{(n-r)!},$ the given expression, $P( 20,5 ),$ evaluates to \begin{array}{l}\require{cancel} =\dfrac{20!}{(20-5)!} \\\\= \dfrac{20!}{15!} \\\\= \dfrac{20(19)(18)(17)(16)(\cancel{15!})}{\cancel{15!}} \\\\= 1,860,480 \end{array}