Each square of the box made from a piece of cardboard must be 4-inches by 4-inches given the starting dimensions of a 12-inch by 16-inch rectangle. A box can be made by using a rectangle by folding a t-shape. The ratio of the length and width confines our t-shape to be 3 squares wide by 4 squares tall which is perfect for a cube. Divide the dimensions by the respective sides square and you will find the answer.