After adding 4 to the numerator of (2/7), how much should be added to the denominator to keep the size of the fraction unchanged?
(2/7)的分子加上4后，要使分数的大小不变，分母应该加多少
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

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

<result>14</result>