You have a set of 20 encyclopedias standing in order on a bookshelf each one containing 100 pages (200 sides but 100 pages). A bookworm eats from the first page of the first volume to the last page of the last volume inclusive.
How many pages did the bookworm eat through?
Answer:
If they are lined up in order then the first page of the first volume is adjacent to the last page of the second volume. The last page of the last volume is adjacent to the first page of the second to last volume. (Look at a set of books on a shelf if you don’t see this.)
1 page from the first volume, one page from the last volume and 100 pages from each of the 18 volumes in between => 1 + 100*18 + 1 = 1802.