A particular cubic box has edge length two inches. If the box has no top, what is the external surface area of the box?
I know that the external surface area of a closed cube is given by the formula 6s^2, where s is the edge length. However, this cube has no top, so it only has five faces instead of six. That means I need to subtract the area of the missing face from the formula. The area of one face of the cube is s^2, so the area of the missing face is 2^2 = 4 square inches. Therefore, the external surface area of the box is 6s^2 - s^2 = 5s^2. Plugging in s = 2, I get 5(2^2) = 5(4) = 20 square inches. 