After adding 4 to the numerator of (2/9), how much should be added to the denominator to keep the size of the fraction unchanged?
(2/9)的分子加上4后，要使分数的大小不变，分母应该加多少
<gadget id="calculator">9 * 3</gadget>
<output>27</output>

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

<result>18</result>