(1) [10 points) Normal distribution. (a) (6 points) Assume the Lifetime X (in hours) of a certain brand of light bulb follows normal distribution with mean n = 160 and variance 02 = 202. Four bulbs are chosen randomly and independently. Compute the probability that none of them has a Lifetime lower than 180 hours. Use (11(1) = 0.8413. (b) (4 points) Let X ~ N (0,02). Compute the fourth moment, E(X4). (Hint: use the moment generating function)