Answer
$_9C_4=126$
Work Step by Step
$_nC_r=\frac{n!}{r!(n-r)!}$
$n=9$ and $r=4$
$_9C_4=\frac{9!}{4!(9-4)!}=\frac{9!}{4!\times5!}$
But, $9!=9\times8\times7\times6\times(5\times4\times3\times2\times1)=9\times8\times\times7\times6\times5!$
$_9C_4=\frac{9\times8\times7\times6\times5!}{4!\times5!}=\frac{9\times8\times7\times6}{4\times3\times2\times1}=\frac{3024}{24}=126$
