(Integer Program) The demand for tickets on Gateway Airlines has grown so much that it intends to buy several new airplanes. There are four types of aircraft from which to choose: the DC-20, the Boeing 797, the Lockheed Bi-Star, and the Eurostar 10. The cost, ca- pacity, and required maintenance time per month for each type of plane are shown in the table below. Gateway wishes to buy the new planes at minimum total cost subject to capacity, maintenance, and training requirements. The new planes must be able to carry a combined total of at least 3400 passengers and have a combined total maintenance requirement of no more than 240 hours per month. In addition, because it is so difficult for workers to become famil- iar with several different types of planes, Gateway does not want to buy more than two different types of planes. The decision on which planes to buy is fur- ther complicated by the fact that there are only five Bi-Stars and seven Eurostars available for purchase. Formulate this problem as a mixed integer program.