Suppose that the length of life in hours (say, X) of the light bulb manufactured by company A is N(800,14400) and the length of life in hours (say, Y) of a light bulb manufactured by company B is N(850,2500). one bulb is randomly selected from each company and is burned unitl "death"
(a) Find the probability that the length of life of the bulb from company A exceeds the length of life of the bulb from company B by at least 15 hours.
(b) Find the probability that at least on eof the bulbs "lives" for at least 920 hours.