If the probability that a fluorescent light has a useful life of at least 800 hours is 0.9, find the probabilities that among 20 such lights
(a) Exactly 18 will have a useful life of at least 800 hours;
(b) At least 15 will have a useful life of at least 800 hours;
(c) At least 2 will not have a useful life of at least 800 hours.