Add 4 to the numerator of (2/3), how much should be added to the denominator to keep the size of the fraction unchanged.
(2/3)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">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">9 - 3</gadget>
<output>6</output>

<result>6</result>