The denominator of (3/4) is enlarged by 5 times, how much should be added to the numerator to keep the size of the fraction unchanged.
(3/4)的分母扩大5倍，要使分数大小不变，分子应加多少．
<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<result>12</result>