Add 21 to the denominator of (3/7), how much should be added to the numerator to keep the size of the fraction unchanged.
(3/7)的分母加上21，要使分数的大小不变，分子应该加上多少．
<gadget id="calculator">7 + 21</gadget>
<output>28</output>

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

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

<gadget id="calculator">12 - 3</gadget>
<output>9</output>

<result>9</result>