Answer: There are 704 non fiction books, 2112 fiction books and 304 reference books.
Explanation:
Let the number of non - fiction books be x
Let the number of fiction books be 3x
let the number of reference books be x-400
Since we have given that
Total number of books in the library = 3120
According to question,
![x+3x+x-400=3120\\\\5x-400=3120\\\\5x=3120+400\\\\5x=3520\\\\x=(3520)/(5)\\\\x=704](https://img.qammunity.org/2020/formulas/mathematics/high-school/26fm0xjq7jqpnww0e5b0ey4i97kwbk2596.png)
Hence, the number of non fiction books = 704
Number of fiction books = 3 × 704=2112
Number of reference books = 704-400=304