There are two layers of books on the bookshelf, with a total of 164 books. If you take 9 books from the lower layer and put them on the upper layer, the number of books on the two layers will be the same. How many books are there in the lower layer than the upper layer?
书架上有两层书，共164本，如果从下层取出9本放到上层去，两层数的本书就相同，原来下层比上层多多少本书．
<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<result>18</result>