There is a really nice math encycolpedia in the library with only 4 volumes and 400 pages each. A hungry worm makes a tiny tunnel from the first page of the first volume to the last page of the last volume. The librarian says to the math teacher:
A worm has pierced 1.600 pages!
Oh! No friend! Not that many! -answers the teacher calculating the result-
Really? so how many pages has the worm pierced?
Can you help the librarian to get the answer?
Translated and adapted from the book "El País de las mates 1" by Miquel Capó
EDITADO: Here is an image that can help you to solve this question: