Math Is Fun Forum

The answer to the cube question is 20.
6 + 8 + 2*6/2 = 6+ 8+ 6 =20
The cube has 6 surfaces, each surface is a plane, this is the first 6 in the equation.
The cube has 8 vertices, each vertice faces a triangle plane, there are 8 triangle plane(8 in the equation)
The cube has 6 surfaces, we see each surface has two crossed planes if we look at the surface ⊥ly ( looks like an X ), however, actually we counted the planes twice, and that's why we need to divide it by 2.