A library has 240 books in a specific section. All the books are either fiction, non-fiction, biography, or poetry.
25% of the books are fiction.
13\displaystyle \frac{1}{3}31 of the books are non-fiction.
The ratio of the number of biography books to the number of poetry books is 3:7 3:7\,3:7
Work out the number of poetry books in this section.
