There is a small bookshelf with upper and lower floors. There are 38 books in the upper layer. After removing 18 books from the lower layer, the number of books in the upper layer is exactly twice the number in the lower layer. How many books are there in the lower layer?
有一个上下两层的小书架．上层有38本书，从下层拿掉18本后，上层的书正好是下层数的2倍，下层原来有多少本书？
<gadget id="calculator">38 / 2</gadget>
<output>19</output>

<gadget id="calculator">19 + 18</gadget>
<output>37</output>

<result>37</result>