The numerator of (3/7) is enlarged by 3 times, how much should the denominator be increased to keep the fraction value unchanged.
(3/7)的分子扩大3倍，要使分数值不变，分母应增加多少．
<gadget id="calculator">3 * 7</gadget>
<output>21</output>

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

<result>14</result>