Question 486201
Combos = Total! / (Subset! * (LeftOver!))
C = 21! / (4! * 17!)
C = (21*20*19*18*(17!) / ((4*3*2*1) * 17!)
C = (7*3 * 5*4 * 19 * 9*2) / (3*4*1*2)
C = 7*5*19*9 = 5985.