A store has two types of animal feed available. Type A contains9 pounds of oats and3 pounds of corn per bag. Type B contains2 pounds of oats and10 pounds of corn per bag. A farmer wants to combine the two types so that the resulting mixture has at least57 pounds of oats and at least75 pounds of corn. The store only has15 bags of type A feed and15 bags of type B feed in stock. Type A costs$4 per bag, and type B costs$4 per bag. How many bags of each type should the farmer buy to minimize his cost?