Answer
35
Work Step by Step
Use the formula of combination: $_{n}$C$_{r}$=$\frac{n!}{r!(n-r)!}$. Plug in 7 for N and 3 for R:
$_{n}$C$_{r}$=$\frac{n!}{r!(n-r)!}$
$_{7}$C$_{3}$=$\frac{7!}{3!(7-3)!}$ -simplify like terms-
$_{7}$C$_{3}$=$\frac{7!}{3! (4!)}$ -write using factorial-
$_{7}$C$_{3}$=$\frac{7*6*5*4*3*2*1}{(3*2*1)(4*3*2*1)}$ -simplify-
$_{7}$C$_{3}$=35
