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?
A sixth of the books is 9900/6 = 
<gadget id="calculator">9900/6</gadget>
<output>1_650</output>
1650 books.
Only 4*1650 = 
<gadget id="calculator">4*1650</gadget>
<output>6_600</output>
6600 books are left.
The library sold 9900-6600= 
<gadget id="calculator">9900-6600</gadget>
<output>3_300</output>
3300 books over the summer.

<result>3_300</result>