This solid cube is built from 27 small cubes. How many of the small cubes cannot be seen at all in the picture?

A small cube is hidden when it has a cube on top of it, a cube in front of it and a cube to its right. That is true for every cube not in the top layer, front layer or right layer: $2\times 2\times 2=8$. (19 is the number you can see.)