In the school library (there are 300 science and technology books and story books), the number of science and technology books is twice that of story books. How many science and technology books are there?

Solution: Suppose there are X story books, then there are 2x science books.

x + 2x = 300

( 1+2)x = 300

3x = 300

3x 3 = 300 x 3

x = 100

A: There are 200 science and technology books.