answer both questions with explanation

3- The electric light bulbs of manufacturer A have a mean lifetime of 1000 hours with a standard deviation of 100 hours. while those of manufacturer B have a mean lifetime of 1200 hours with a standard deviation of 200 hours. If random samples of 100 bulbs of manufacturer A and 50 each brand are tested, what. is the probability that the brand A bulbs will have a mean lifetime that is at least (a) 160 hours. (b) 250 hours more than the brand B bulbs