There are two layers of books on the bookshelf, 64 books on the upper layer and 80 books on the lower layer. When how many books are given to the lower layer by the upper layer, the number of books on the lower layer is three times that of the upper layer?
书架上有两层书，上层64本，下层80本，当上层给下层多少本时，下层的本数是上层的3倍？
<gadget id="calculator">64 + 80</gadget>
<output>144</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">144 / 4</gadget>
<output>36</output>

<gadget id="calculator">64 - 36</gadget>
<output>28</output>

<result>28</result>