Multiply the denominator of the fraction (7/(a)) by 3, how much should be added to the numerator to keep the original fraction unchanged
给分数(7/(a))的分母乘以3，要使原分数大小不变，分子应加上多少
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">21 - 7</gadget>
<output>14</output>

<result>14</result>