How 4 gallons of milk make 54 one-cup servings?

Answer 1

4 gallons = 512 fluid ounces

If you're serving it in standard 8-ounce cups, then it makes 64 of them.

If you only get 54 cups out of it, then somebody has slipped in a bunch of 9.5-ounce cups.

Answer 2

Since each gallon of milk contains 16 cups, 4 gallons will give you a total of 64 cups. To make 54 one-cup servings, you will use 54 cups out of the 64 cups available from the 4 gallons of milk.