The numerator of (2/7) is increased by 6, how much should the denominator be increased to keep the size of the fraction constant.
(2/7)的分子增加6，要使分数的大小不变，分母应增加多少．
<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>