Tasty Bakery sells three kinds of muffins: chocolate chip muffins at 65 cents each, oatmeal muffins at 70 cents each, and cranberry muffins at 75 cents each. Charles buys some of each kind and chooses 3 times as many chocolate chip muffins as blueberry muffins. If he spends $6.85 on 10 muffins, how many oatmeal muffins did he buy