A 5 volume set of book sit on a shelf, in order. Each volume is 1 inch thick.
A bookworm eats its way from the front cover of the first volume to the back cover of the last volume. How far did the bookworm travel?
3 inches.
Proof
The front of volume 1 is directly next to volume 2 when arranged on the shelf. Likewise, the back of volume 5 is next to volume 4.
Thus the bookworm does not eat through volume 1 and 5, only 2, 3 and 4. It travels 3 books, thus 3 inches.