Problem 4 (5 pts): How many ways are there to select 6 cakes from 20 varieties (a): if there are no two cakes from the same variety? (b): if all cakes are of the same variety? (c): if the half of them are from one variety and the other half are from a different verity? (d): if there are 3 cakes from one variety, 2 cakes form another variety, and one from yet another variety. Note: the order is which specific cakes are being selected does not matter