1 1. A newly replaced light bulb is used until it burns out and then is replaced immediately by another. The two bulbs are selected randomly from a large lot of bulbs that the manufacturer says have an approximately normal lifetime distribution with a mean of 800 hours and a standard deviation of 100 hours. a. Describe the distribution of the total lifetime of the two bulbs used in sequence. b. What is the probability that the sum of the two lifetimes exceeds 1800 hours? c. What is the probability that the sum of the lifetimes of nine such bulbs selected at random and burned sequentially exceeds 8100 hours? d. What is the probability that the second light bulb in the sequence burns longer than the rst