the If the probability that fluorecent light has a useful life of at least 500 hours is 0.85, find the probabilities that among 20 such lights
(a) 18 will have a useful life of at least 500 hours;
(b) At least 15 will have a useful life of at least 500 hours;
(c) At least two will not have a useful life ot at least 500 hours