a) Let X be the time until the next bus arrival. The time between two bus arrivals is

exponential with mean 10 minutes. You come to the bus stop and see a friend waiting. You ask

her how long she has been waiting, and she answers, 6 minutes. What is the expected time until

the bus arrival?

b) Now, suppose that bus arrivals happen deterministically every 10 minutes. How does your

answer change?