Initial measure 63 inches
After dividing the sandwich by 3, the measure of one piece is 21 inches
After dividing one of the previous pieces of the sandwich by 3, the measure of one piece is 7 inches
After dividing one of the previous pieces of the sandwich by 2, the measure of one piece is 7/2 inches
Therefore, the longest piece measures 21 inches and the smallest piece measures 7/2. Subtracting 7/2 from 21, we have: