At an underground station, trains arrive according to a Poisson process of rate twenty per

hour.

(a) Suppose that I arrive at the platform and intend to take the first westbound train

that arrives. What is the expected time I have to wait until the first train arrives?

(b) After ten minutes, no train has arrived and I am still on the platform. What is the

expected further time I have to wait until the first train arrives?