23. A flashlight needs two batteries to be operational. Consider such a flashlight along with a set of n functional batteries—battery 1, battery 2,... , battery n.

Initially, battery 1 and 2 are installed. Whenever a battery fails, it is immediately replaced by the lowest numbered functional battery that has not yet been put in use. Suppose that the lifetimes of the different batteries are independent exponential random variables each having rate μ. At a random time, call it T , a battery will fail and our stockpile will be empty. At that moment exactly one of the batteries—

which we call battery X—will not yet have failed.

(a) What is P{X = n}?

(b) What is P{X = 1}?

(c) What is P{X = i}?

(d) Find E[T ].

(e) What is the distribution of T ?