Question

${}^{10}{p}_r=5040$, then r =

Answer 1

${}^n{p}_x=\frac{n!}{(n-x)!}$

${}^{10}{p}_r=10\times 9\times 8\times 7$

${}^{10}{p}_r={}^{10}{p}_6$

$r=6$