question: 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*1=2>>2 inches from each dimension
  So each box has 5-2=<<5-2=3>>3 inch width
  It also has a 6-2=<<6-2=4>>4 inch height
  Finally, it has a 4-2=<<4-2=2>>2 inch depth
  So the inner volume of one box is 4*3*2=<<4*3*2=24>>24 cubic inches
  So in total the inner volume of the 3 boxes is 3*24=<<3*24=72>>72 cubic inches
  #### 72