At the post office, patrons wait in a single line for the first open window. An average of 100 patrons per hour enter the post office, and each window can serve an average of 45 patrons per hour. The post office estimates a cost of £0.1 for each minute a patron waits in line and believes that it costs £20 per hour to keep a window. Inter-arrival times and service times are independent and exponentially distributed.