7 An average of 50 customers per hour arrive at a small post office. Interarrival times are exponentially distributed.

Each window can serve an average of 25 customers per hour. Service times are exponentially distributed. It costs

$25 per hour to open a window, and the post office values the time a customer spends waiting in line at $15 per customer-hour. To minimize expected hourly costs, how many postal windows should be opened?