Astore has two types of animal feed available. type a contains 9 pounds of oats and 3 pounds of corn per bag. type b contains 2 pounds of oats and 10 pounds of corn per bag. a farmer wants to combine the two types so that the resulting mixture has at least 57 pounds of oats and at least 75 pounds of corn. the store only has 15 bags of type a feed and 15 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?