Put a label around it, that is, calculate the surface area of a cuboid with a length of 20cm, a width of15cm and a height of 30cm.

The surface area of a cuboid =2* length * width +2* length * height +2* width * height = 2 * 20 *15+2 *15 * 30 = 2700 (cm).

If it is not covered, the area of a top rectangle is missing, that is, = length * width +2* length * height +2* width * height = 20 *15+2 * 20 * 30+2 *15 * 30 = 2400 (cm).