Free Doughnuts

Situation: You have a golden opportunity to take as many doughnut holes as you can carry in a box, unfortunately, you don't have a box to carry them. All you have is a piece of cardboard that is 24 inches by 15 inches. You remember something from math class about making a box by cutting a square out of each corner and folding up the sides. You decide that you will do that and make the biggest box possible (without a lid).

Your problem is to figure out how much to cut out of each corner to make the box with the biggest (maximum) volume.