A store has two types of animal feed available. Type A contains 3 pounds of oats and 3 pounds of corn per bag. Type B contains 1 pound of oats and 7 pounds of corn per bag. A farmer wants to combine the two types so that the resulting mixture has at least 18 pounds of oats and at least 54 pounds. The store only has 11 bags of type A feed and 12 bags of type B feed in stock. type A costs $4 per bag, and type B costs $1 per bag. how many bags of each type should the farmer buy to minimize his cost?