At a post office, customers wait in a single line for the first open window. An average of 100 customers per hour enter the post office, and each window can serve an average of 25 customers per hour. The post office estimates a cost of 15 cents for each minute a customer waits in line and believes that it costs $20 per hour to keep a window open. Interarrival times and service times are exponential. To minimize the total expected hourly cost, how many windows should be open?