Because science and technology books are 4/5 of literature books, that is, science and technology books-4/5 literature books,

The sum of science and technology books and story books is twice that of literature and art, that is, science and technology books+story books =2* literary books.

So the conclusion is: 4/5 literature book+story book =2* literature book, and the solution is: 6/5 literature book = story book.

There are 750 story books, so there are 750 literature books = 5/6 = 625.

So there are 625*4/5=500 science and technology books.