Answer
$_4P_0 = 1$
Work Step by Step
RECALL:
$_nP_r = \dfrac{n!}{(n-r)!}$
Use this definition to have:
$\require{cancel}
_4P_0=\dfrac{4!}{(4-0)!}
\\_4P_0=\dfrac{4!}{4!}
\\_4P_0 = 1$
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