Add 4 to the numerator of (4/9), how much should be added to the denominator to keep the size of the fraction unchanged.
(4/9)的分子加上4，要使分数的大小不变，分母应加上多少．
<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<gadget id="calculator">18 - 9</gadget>
<output>9</output>

<result>9</result>