0! = 1
1! = 1= 1
2! = 1*2= 2
3! = 1*2*3= 6
4! = 1*2*3*4= 24

Поэтому
С 3 15 = 15!\[ 3!(15-3)! ] = (12! *13*14*15)/(1*2*3*12!)=
13*14*15/(1*2*3)