Add 6 to the numerator of (2/7), how much should be added to the denominator to keep the size of this fraction constant.
(2/7)的分子加上6，要使这个分数的大小不变，分母应该加上多少．
<gadget id="calculator">2 + 6</gadget>
<output>8</output>

<gadget id="calculator">8 / 2</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>