An airline company maintains a schedule of two daily flights between Salt Lake CityDallas and Chicago. A crew that leaves a city in the morning has to return there at night. The crew can be brought back on another airline. There are $6$ airplanes in use. When a crew is flying, the cost is $$200$ per hour. When a crew is waiting or being flown back, the cost is $$75.$ How should the company schedule its crews to minimize cost?