10 people enter elevator with 40 floors above. Suppose that each person chooses a floor to exit with equal probability and selection of each person doesn't affect that of the others.
(a) Determine the probability that either exactly 4 ppl get out on floor 35 or exactly 4 ppl get out on floor 36 (inclusive OR)?
(b) Determine the probability that the elevator stops on floors 11, 15, and 39?