試題詳解

16. (5%) Kim is at a train station, waiting to make a phone call. Two public telephone booths, next to each other, are occupied by two callers, and 1 1 persons are waiting in a single line ahead of Kim to call. If the duration of each telephone call is an independent exponential random variable with λ= 1/3, what is the variance of Kim's waiting time?Kim's waiting time is defined as the time interval from now to the moment that a phone is available for Kim to make his call.
(A) 18
(B) 27
(C) 99/4
(D) 99
(E) 108