A sixth of the 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.

# Answer

3300
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
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?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
3300
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?
train