An FBO (a "fixed-base-operator", usually the manager of a small-town airport) makes his revenue in three ways: flight instruction, charter flights, and aircraft maintenance. These activities earn revenues of $25/hour, $40/hour, and $35/hour respectively. The FBO has enough work to guarantee at least 40 hours of work per week. He also, for reasons of sanity and family, refuses to work more than 60 hours per week. He has at least ten hours of each activity to do each week, but refuses to more than twenty hours of any one of these activities to keep himself from getting bored with any one activity. How should he schedule his time to optimize his revenue?