The numerator of (8/13) is multiplied by 2, how much should be added to the denominator to keep the size of the fraction constant.
(8/13)的分子乘以2，要使分数的大小不变，分母应加上多少．
<gadget id="calculator">13 * 2</gadget>
<output>26</output>

<gadget id="calculator">26 - 13</gadget>
<output>13</output>

<result>13</result>