The staircase is built from identical cubes (every row of the staircase is the same, 3 rows deep). How many cubes can NOT be seen in this picture?

There are $3\times (4+3+2+1)=30$ cubes. A cube is hidden when it has a cube above it, a cube in front of it and a cube to its right. In each of the 2 back rows that is $3+2+1=6$ cubes, so $2\times 6=12$ are hidden (18 can be seen).