The bookshelf has upper and lower floors. There are 450 books on the upper floor, which is 10 more than twice as many books on the lower floor. How many books are there on the lower floor?
书架有上、下两层，上层有图书450本，比下层的2倍多10本，下层有图书多少本？
<gadget id="calculator">450 - 10</gadget>
<output>440</output>

<gadget id="calculator">440 / 2</gadget>
<output>220</output>

<result>220</result>