The correct option is B 3 10_pr = 720 ⇒10!/(10 r!)! =720 ⇒10!/(10 r)!=6! ⇒ 5040=(10 r)! ⇒ 7!=(10 r)! ⇒ 7=10 r ⇒ r=3 ...

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Finding Squares of Numbers

If 10Pr=720...

Question

If $^{10} P_{r} = 720$ , then $r =$

2

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Solution

The correct option is B $3$
$10_{p r}$ = 720
$\Rightarrow \frac{10!}{(10 - r!)!}$ =720
$\Rightarrow \frac{10!}{(10 - r)!} = 6!$
$\Rightarrow 5040 = (10 - r)!$
$\Rightarrow 7! = (10 - r)!$
$\Rightarrow 7 = 10 - r$
$\Rightarrow r = 3$

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