Question

Evaluate: $\frac{n!}{r!(n-r!)},$ when $n=5,r=2$

Answer 1

$\frac{n!}{r!(n-r!)}=\frac{5!}{2!(5-2)!}(\because n=5,r=2)$
$=\frac{5!}{2!3!}=\frac{5\times 4\times 3!}{2\times 1\times 3!}=\frac{5\times 4}{2}=10$