Tasty Bakery sells three kinds of muffins chocolate chip muffins at 30 cents each, oatmeal muffins at 35 cents each, and blueberry muffins at 40 cents each. Charles buys some of each kind and chooses three times as many blueberry muffins as chocolate chip muffins. If he spends $4.75 on 13 muffins, how many oatmeal muffins did he buy?