## Thinking Mathematically (6th Edition)

$\displaystyle \frac{n!}{(n-r)!}$
See: Permutations Formula, page 698. The number of permutations possible if $r$ items are taken from $n$ items is ${}_{n}P_{r}=\displaystyle \frac{n!}{(n-r)!}$. ( $n!=n(n-1)(n-2)\cdots(3)(2)(1)$ )