A company gives transportation service to tourist groups by renting busses as needed. Business arrangements made in advance indicate that number of busses needed for the next 4 months is estimated at 10, 9, 15, and 8, respectively. This company does not own any buses but rents the buses as needed. The company can rent a bus for one month at a price of $ 14, and for two months at a price of $ 26. The company currently has no buses. Let the decision variables be defined as written down below: Decision Variables X1: Number of buses, rented for one month, starting from month 1 X2: Number of buses, rented for one month, starting from month 2 X3: Number of buses, rented for one month, starting from month 3 X4: Number of buses, rented for one month, starting from month 4 X5: Number of buses, rented for two months, starting from month 1