COMBINATION is the number of ways that a certain number of objects, r, can be selected out of a total number of objects, n.
nCr: selecting ‘r’ objects out of ‘n’ total objects.
4C0 = 1 way, picking nothing
4C1 = 4 / 1 = 4 ways, each of the 4 objects
4C2 = (4*3) / (2*1) = 6
4C3 = (4*3*2) / (3*2*1) = 4
4C4 = (4*3*2*1) / (4*3*2*1) = 1