There are 130 books on the upper shelf and 68 books on the lower shelf. How many books are taken from the upper shelf and placed on the lower shelf, so that the number of books on the two floors is the same?
上层书架有130本书，下层书架有68本书，从上层书架取出多少本放到下层书架，两层的本数就一样多了？
<gadget id="calculator">130 - 68</gadget>
<output>62</output>

<gadget id="calculator">62 / 2</gadget>
<output>31</output>

<result>31</result>