Baking a tray of corn muffins takes 4 cups of milk and 3 cups of wheat flour. Baking a tray of bran muffins takes 2 cups of milk and 3 cups of wheat flour. A baker has 20 cups of milk and 21 cups of wheat flour. He makes ​4$ profit per tray of corn muffins and ​3$ profit per tray of bran muffins. How many trays of each type of muffin should the baker make to maximize his​ profit?