The denominator of (5/8) is expanded to 32, how much should be added to the numerator to keep the size of the fraction unchanged.
(5/8)的分母扩大到32，要使分数大小不变，分子应该加上多少．
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<gadget id="calculator">20 - 5</gadget>
<output>15</output>

<result>15</result>