CDP is a good source of practice for the cube counting on the real test. The real thing has fewer cubes in each figure than that in CDP. In this problem, if you look at the right side you will see a line diagonally cutting through a cube. Since it is not possible for that to happen, you must know that the area has an overlap and the right half of that cube is in reality the cube that is half hidden. Looking at the height of that cube and comparing it to the ones on it left, you must realize that that cube in a one level lower than the one on the left. Hope this helps in somewhat understading the problem