Question 19
There are 165 unit cubes arranged in 5 square layers with no space between the cubes as shown. The layers are 1 by 1, 3 by 3, 5 by 5, 7 by 7, and 9 by 9. If a sixth layer with the same pattern is added to the bottom of the figure, how many unit cubes are completely surrounded by six other unit cubes?
