Question: From a hundred used cars sitting on a lot, 20 are to be selected for a test designed to check certain safety requirements. These cars will then be put back onto the lot and, again, 20 will be selected for a test designed to check antipollution standards.
(a) In how many ways can the first selection be made?
(b) In how many ways can the second selection be made?
(c) In how many ways can both selections be made?
(d) In how many ways can both selections be made if exactly five cars are to undergo both tests?