The numerator of (3/7) is increased by 9, how much should the denominator be increased to keep the size of the fraction constant.
(3/7)的分子增加9，要使分数的大小不变，分母应增加多少．
<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>