The numerator of (4/5) is increased by 12, if the size of the fraction remains the same, how much should the denominator be increased.
(4/5)的分子增加12，要是分数的大小不变，分母应增加多少．
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

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

<result>15</result>