Answer
$5$
Work Step by Step
Using $P(n,r)=\dfrac{n!}{(n-r)!},$ the given expression, $P(
5,1
),$ evaluates to
\begin{array}{l}\require{cancel}
=\dfrac{5!}{(5-1)!}
\\\\=
\dfrac{5!}{4!}
\\\\=
\dfrac{5(4!)}{4!}
\\\\=
\dfrac{5(\cancel{4!})}{\cancel{4!}}
\\\\=
5
\end{array}