There are a number of science and technology trees and literature books in the library, the number ratio is 5:7. Later, I bought 15 literary books, and the quantity ratio was 2:3. How many books are there? There is a solution than an answer. Method 1: I bought 15 literature books later, but the number of science books is still the same? Books on literature and art used to account for 7/ 12 of the total, but now they account for 3/5, 3/5-7/ 12 = 1/60, which is the corresponding scoring rate of 15 books on literature and art. 15÷ 1/60=900 books, which is the total number of original books.

? Comprehensive formula: 15 \( 3/5-7/ 12)

? = 15÷ 1/60

? =900 (Ben)

The second method: solution: let a * * * have X books.

? 3/5x—7/ 12x= 15

1/60x = 15

x？ = 15÷( 1/60)

? x？ =900

? In proportion, science and technology books are 900*5/ 12=375, and literature and art books are 900*7/ 12=525.