Answer
$n=11$
Work Step by Step
$_nP_r=\frac{n!}{(n-r)!}$, hence here the equation is: $\frac{n!}{(n-4)!}=8\frac{n!}{(n-3)!}$
If we multiply both sides by $(n-3)!=(n-3)\cdot(n-4)!$ we get: $n!(n-3)=8n!\\n-3=8\\n=11$
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