Add 9 to the numerator of (3/7), how much should be added to the denominator if the size of the fraction is to remain the same
(3/7)的分子加上9，如果要使分数的大小不变，分母应该加上多少
<gadget id="calculator">3 + 9</gadget>
<output>12</output>

<gadget id="calculator">12 / 3</gadget>
<output>4</output>

<gadget id="calculator">7 * 4</gadget>
<output>28</output>

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

<result>21</result>