Automobiles arrive at the drive through window at the downtown Urbana, Illinois, post office at the rate of 4 every 10 minutes (or .4/min)= lambda. The average service time is 2 minutes (.5/min)=mu . The Poisson distribution is appropriate for the arrival rate and service times are exponentially distributed.
I'm just having trouble on the last sub-section h) If a second drive through window, with its own server, were added, the average time a car is in the system = blank minutes, please round to 2 decimal places