Below is an instruction that describes a task. Write a response that appropriately completes the request in English. Please answer in English.

### Instruction:
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?

### Response:
A sixth of the library books is 9900/(6)=<<9900/(6)=1650>>1650 books.
Only 4*1650=<<4*1650=6600>>6600 books are left.
The library sold 9900-6600=<<9900-6600=3300>>3300 books over the summer.
#### 3300