Answer:
672
Explanation:
If we call the number of non-fiction books as x, the number of fiction books would be 3x. Therefore: we can write the following equation:
3x - 120 = 2(x - 24) ← the 3x - 120 and x - 24 represents the new number of books
3x - 120 = 2x - 48
x - 120 = 48
x = 168 which means 3x = 3 * 168 = 504, therefore the total number of books is 168 + 504 = 672.