13.009. In the library, there are books in English, French, and German. English books make up $36 \%$ of all foreign language books, French books make up $75 \%$ of the English books, and the remaining 185 books are German. How many foreign language books are there in the library?
## Solution.

Let $x$ be the total number of books in foreign languages. The number of English books is $0.36x$; the number of French books is $0.75 \cdot 0.36x$. According to the problem, $0.36x + 0.75 \cdot 0.36x + 185 = x$, from which we find $x = 500$.

Answer: 500