There are 90 science and technology books suitable for primary school students, accounting for 1/2 of science and technology books, so all science and technology books are 90÷( 1/2)= 180.

Science and technology books account for 3/8 of all books, so all books are180/3/8) = 480.

There are 480 kinds of books purchased by Xinhua Bookstore.

The comprehensive formula is 901/2/3/8 = 480 copies.