Question: Suppose that in a particular application requiring a single battery, the mean lifetime of the battery is 4 weeks, with a standard deviation of 1 week. The battery is replaced by a new one when it dies, and so on. Assume lifetimes of batteries are independent. what approximately, is the probability that more than 26 replacements will have to be made in a two-year period, starting at the time of installation of a new battery. And not counting that new battery as a replacement? [Hint: Use the normal approximation to the distribution of the total lifetime of n batteries for a suitable n.]