Answer
126
Work Step by Step
Use the formula of combination: $_{n}$C$_{r}$=$\frac{n!}{r!(n-r)!}$. Plug in 9 for N and 4 for R:
$_{n}$C$_{r}$=$\frac{n!}{r!(n-r)!}$
$_{9}$C$_{4}$=$\frac{9!}{4!(9-4)!}$ -simplify like terms-
$_{9}$C$_{4}$=$\frac{9!}{4! (5!)}$ -write using factorial-
$_{9}$C$_{4}$=$\frac{9*8*7*6*5*4*3*2*1}{(4*3*2*1)(5*4*3*2*1)}$ -simplify-
$_{9}$C$_{4}$=126
