GamesNovatory/Enigmas/How Much (How Many) ?/The Worm

The original puzzle
What distance does a worm travel from the first page of Volume 1 to the last page of Volume 3 in an Encyclopedia on the book-shelf ? The pages of each volume are 10 centimetres thick and each cover is 5 millimeters thick.

The Solution
One must never forget how books are placed on a shelf, from left to right,
     with each book inverted, the end on the left and the beginning on the right.

The worm thus travels through -
     the front cover of Book 1, all of Book 2, and the back cover of Book 3,
          4 covers (2 centimetres) and one set of pages (10 centimetres).