The number of science and technology books purchased by school libraries is three-seventh of that of story books, and the number of science and technology books and story books purchased is 1500. How many copies? Solution: Suppose there are X story books and 3/7x science books.

x+3/7x= 1500

10/7x= 1500

x= 1050

1500- 1050=450 volumes

A: There are 450 science and technology books.

Solution 2: 1500×3/(7+3)=450 sci-tech books, accounting for 3/(7+3)=3/ 10 of the total number of two books.