How do you, wright 50+30z=250+10z the answer z=10 in a story?

Answer: a school library has a book shelf with 30 cells, each cell contains a certain amount of books, and an additional 50 books outside the shelf. Another school has a book shelf with 10 cells, each cell containing a certain amount of books, and an additional 250 books outside the book shelf. Assuming all books between the two schools have the same dimensions, find the number of books each cell must contain if both schools have equal amount of books in their library.