A fast-food restaurant has one drive-through window. On average, 40 customers arrive per hour at the window. It takes an average of one minute to serve a customer. Assume that inter-arrival and service times are exponentially distributed.

a. On average, how many customers are waiting in line?

b. On average, how long does a customer spend at the restaurant (from time of arrival to time service is completed)?

c. What fraction of the time are more than three cars in line? (Here, the line includes the car, if any, being serviced.)