The library has 9,900 books. Over the summer, they sold some books. If only four sixths of the books are left, how many did they sell?
Reason slowly and give your answer.
3300
A sixth of the books is 9900 / 6 = 1650 books.

Only 4 * 1650 = 6600 books are left.

The library sold 9900 - 6600 = 3300 books over the summer.

.

So, the final answer is 3300