Add 12 to the numerator of (3/7), how much should be added to the denominator to keep the size of the fraction unchanged.
(3/7)的分子加上12，要使分数的大小不变，分母应加上多少．
<gadget id="calculator">7 * 5</gadget>
<output>35</output>

<gadget id="calculator">35 - 7</gadget>
<output>28</output>

<result>28</result>