Automobiles arrive at the drive - through window at the downtown Baton Rouge , Louisiana , post office at the rate of 4 every 10 minutes . The average service time is 1.0 minutes . The Poisson distribution is appropriate for the arrival rate and service times are negative exponentially distributed .g) The probability that there are exactly 2 cars in the system = (round your response to three decimal places). h) If a second drive-through window, with its own server, were added, the average time a car is in the system = minutes (round your response to two decimal places).