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

### 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:
 พวกเขาขาย 6/6 - 4/6 = <<6/6-4/6=2/6>>2/6 ของหนังสือ
พวกเขาขาย 2/6 * 9,900 = <<2/6*9900=1650>>1650 เล่ม