After borrowing three-quarters of science books and two-thirds of literature books, there are so many books left.

Science and technology books × (1-3/4) = literature and art books × (1-2/3)

Science and technology books: literature books = (1-2/3): (1-3/4) = 4: 3.

Science and technology books are

3500× (4+3) 4 = 2000 (Ben)

Literary books are

3500-2000 = 1500 (Ben)