Answer: There are 704 non fiction books, 2112 fiction books and 304 reference books.
Step-by-step 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,
[tex]x+3x+x-400=3120\\\\5x-400=3120\\\\5x=3120+400\\\\5x=3520\\\\x=\frac{3520}{5}\\\\x=704[/tex]
Hence, the number of non fiction books = 704
Number of fiction books = 3 × 704=2112
Number of reference books = 704-400=304