A cafeteria serving line has a coffee urn from which customers serve themselves. Arrivals at the urn follow a Poisson distribution at the rate of three per minute. In serving themselves, customers take about 15 seconds, exponentially distributed.

f.f the cafeteria installs an automatic vendor that dispenses a cup of coffee at a constant time of 15 seconds, how long would you expect it to take (in minutes) to get a cup of coffee, including waiting time?