17. On a certain day, the first customer of a single-teller bank arrives at 9:00 A.M. when the bank opens. The second customer arrives 10 minutes later at 9:10 A.M. Suppose that the service times of customers are independent exponential random variables each with mean 10 minutes, customers wait in a single queue if the teller is busy serving others, and they leave the bank as soon as they are served. Find the probability density function and the expected value of the amount of time the second customer will spend in the bank.