There are 180 books on the upper shelf, and 240 books on the lower shelf. How many books are taken from the upper shelf and placed in the lower shelf, so that the number of books on the lower shelf is exactly twice that of the upper shelf.
上层书架有180本书，下层有240本书，从上层取出多少本书放入下层，可以使下层的书正好是上层的两倍．
<gadget id="calculator">180 + 240</gadget>
<output>420</output>

<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">420 / 3</gadget>
<output>140</output>

<gadget id="calculator">180 - 140</gadget>
<output>40</output>

<result>40</result>