Answer
$20$
Work Step by Step
$P(n,r)=\displaystyle \frac{n!}{(n-r)!}$.
Factorials:
$n!=n(n-1)!=n(n-1)(n-2)!=...$
$n!=n(n-1)(n-2)\cdots(3)(2)(1)$
$ 0!=1,\qquad$ by definition.
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$P(5,2)=\displaystyle \frac{5!}{(5-2)!}=\frac{5\times 4\times 3!}{3!}$
$=5\times 4=20$