Add 15 to the numerator of (5/7), how much should be added to the denominator to keep the size of the fraction unchanged.
(5/7)的分子加上15，要使分数的大小不变，分母应加上多少．
<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<gadget id="calculator">28 - 7</gadget>
<output>21</output>

<result>21</result>