Right now, the ratio of nonfiction to fiction is 9:18. If this is reduced, it is a ratio of 1:2, so there needs to be twice as many fiction books as nonfiction.
If we add 6 fiction books, we have 24 fiction books. We can now set up this equation:
Now we cross multiply
2x=24
We can solve for x by dividing both sides by two, so:
x = 12. There will be 12 nonfiction books.