Answer
$24$
Work Step by Step
We know that $P(n,r)=\dfrac{n!}{(n-r)!}.$
Hence,
$P(4,4)=\dfrac{4!}{(4-4)!}=\dfrac{4\cdot3\cdot2\cdot1}{0!}=4\cdot3\cdot2\cdot1=24$
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