By looking at the 6 individual surfaces of the original block, all the 1cm cubes along the perimeter of each surface will have more than 1 painted surface after being cut out. For exmaple, block C as shown in the diagram has 3 painted sides after being cut out, while block B has 2 and block A having only 1. The calculation shown below is simply a calculation of how many blocks there are that are similar to block A.