There are 200 books on the lower shelf, which is 40 fewer than the upper shelf. The total number of books on the upper and lower shelves is twice that of the middle shelf. How many books are there on the middle shelf?
书架下层有200本书，比上层少40本，上、下两层书的总和是中间一层的2倍，中间一层有书多少本？
<gadget id="calculator">200 + 40 + 200</gadget>
<output>440</output>

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

<result>220</result>