Add 15 to the denominator of (2/5) to keep the size of the original fraction unchanged, how much should be added to the numerator.
(2/5)的分母加上15，使原分数大小不变，分子应加上多少．
<gadget id="calculator">2 * 4</gadget>
<output>8</output>

<gadget id="calculator">8 - 2</gadget>
<output>6</output>

<result>6</result>