The junior class has $300 to invest in a candy sale. They can sell no more than 9 cases of suckers and they must sell at least 18 cases of M & M's. Suckers cost $4 a case and M & M's cost $12 a case. If the profit on a case of suckers is $10 and the profit on a case of M & M's is $20, how many each should they buy to maximize their profit?