Add 4 to the numerator of (2/7), how much should be added to the denominator to keep the size of the original fraction unchanged.
(2/7)的分子加上4，要使原分数大小不变，分母应加上多少．
<gadget id="calculator">4 + 2</gadget>
<output>6</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">21 - 7</gadget>
<output>14</output>

<result>14</result>