A rectangular prism has four lateral surfaces. If the sides of the base are L and W, two of the lateral surfaces are rectangles of side L and H, and the other two are rectangles of sides W and H. Then, the lateral area is the sum of the areas of the four lateral rectangles: 2 * L * H + 2 * W * H= 10 * 4 = 2 * 10 * 4 + 2 * 5 * 4 = 2*40 + 2 * 20 = 80 + 40 = 120.
