In this five-step staircase of cubes, three cubes wide, how many cubes are completely hidden in the picture?

A cube is hidden when there is a cube above it, in front of it and to its right, so only the left two cubes of each row can be hidden. In the back row (5 high) the cubes level with the next step, 4, are hidden: 4 per column; then 3, 2 and 1 in the following rows. Two columns: $2\times (4+3+2+1)=20$.