Question: Ships arrive at a harbor at the rate of one every 60 ± 30 minutes. There are six berths to accommodate them. They also need the service of a crane for unloading and only one crane is available. After unloading, 10% of the ships stay for refuel before leaving, while the others leave immediately. Ships do not require the use of crane for refueling. It takes 7 ± 3 hours for unloading and 60 ± 20 minutes for refueling. Assume that the crane is subjected to routine maintenance once in every 100 hours, and it takes 5 ± 2 hours to complete the maintenance. The crane's unloading operation is not interrupted for maintenance. The crane is taken for maintenance as early as possible after completing the current unloading activity. Simulate the system for unloading 500 ships that require refueling.