Question: I need help with this problem:
John has 3 boxes.  Each box is 5 inches by 6 inches by 4 inches.  The walls are 1 inch thick.  What is the total inner volume of all 3 boxes?
Answer: 
The walls subtract 2*1=2 inches from each dimension
So each box has 5-2=3 inch width
It also has a 6-2=4 inch height
Finally, it has a 4-2=2 inch depth
So the inner volume of one box is 4*3*2=24 cubic inches
So in total the inner volume of the 3 boxes is 3*24=72 cubic inches