An amazon warehouse has three different drones to deliver products to its customers. The drone A can deliver 10 products per hour, at operating cost of 2$ per hour. The drone B can deliver 18 products per hour, at operating cost of 5$ per hour. The drone C can deliver 15 products per hour, at operating cost of 3$ per hour. The maintenance times per hour of operation are 10 minutes, 15 minutes, and 12 minutes respectively for drones A, B and C. The technicians can only spend 500 minutes per day to maintain these drones. Each drone can be operated for a maximum of 15 hours per day. If the warehouse must deliver at least 500 products per day, how many hours should the manager operate these drones to minimize operating costs?