Add 27 to the numerator of 1(2/7), how much should be added to the denominator to keep the size of this fraction unchanged.
1(2/7)的分子加上27，要使这个分数的大小不变，分母应加上多少．
<gadget id="calculator">28 - 7</gadget>
<output>21</output>

<result>21</result>