The numerator of (2/7) is multiplied by 4, how much should be added to the denominator to keep the size of the fraction constant
(2/7)的分子乘以4，要使分数的大小不变，分母应加上多少
<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<gadget id="calculator">28 - 7</gadget>
<output>21</output>

<result>21</result>