The denominator of (5/8) is enlarged by 3 times, how much should be added to the numerator to keep the size of the fraction unchanged.
(5/8)的分母扩大了3倍，要使分数的大小不变，分子应加上多少．
<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<gadget id="calculator">15 - 5</gadget>
<output>10</output>

<result>10</result>