4). At the Post Office, customers wait in a single line for the first open window. Customers enter the post office at an average of 100 per hour, and each window can serve an average of 45 people per hour. The post office estimates a cost of $10 for each minute that a customer waits in line and believes that it costs$30 per hour to keep a window open. What is the minimum number of windows that must be open to prevent the line from growing infinitely long? What is the optimal number of windows to keep open in order to minimize total hourly cost? What is the total hourly cost to operate at the optimal number of windows? (Round your answer to the nearest whole dollar.)