The ratio of original books in school A and school B is 7 to 5. If school A gives school B 1300 copies, the number of books in school A and school B is 3 to 4. A How many schools are there?
Solution: Set up a school with 7x original books and a school with 5x original books.
(7x- 1300)/(5x+ 1300)= 3/4
3(5x+ 1300)= 4(7x- 1300)
15x+3900=28x-5200
13x=9 100
x=700
7x=4900
There used to be 4,900 books in a school.