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? Let's think first. Chain of thought:
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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.
Therefore, the answer is 3300.
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