It turns out that the story books in the school library account for 3/5 of the total collection. Recently, the school bought 400 story books. At this time, the story books in the library account for 2/3 of the total number of existing books. How many books are there in the library now?

400÷( 1-3/5)÷( 1-2/3)

= 1000÷ 1/3

=3000 copies