Suppose that X, the waiting time before the first customer comes to the university computer help desk, follows an exponential distribution with mean 8 minutes.

(a) What's the probability that the first person comes exactly 20 minutes after the help desk opens?

(b) What's the probability that the help desk will wait for more than 12 minutes before the first customer comes?

(c) Find P(X > 20 | X > 8) , the probability that the help desk will wait for more that 25 minutes given that no one comes

to the help desk in the first 8 minutes?