Add 4 to the numerator of (2/9), how much should be added to the denominator to keep the fraction constant.
把(2/9)的分子上加上4，要使这个分数大小不变，分母应加上多少．
<gadget id="calculator">9 * 3</gadget>
<output>27</output>

<gadget id="calculator">27 - 9</gadget>
<output>18</output>

<result>18</result>