Add 18 to the denominator of (4/9), how much should be added to the numerator to keep the size of the fraction unchanged.
(4/9)的分母加上18，要使分数大小不变，分子应该加上多少．
<gadget id="calculator">18 + 9</gadget>
<output>27</output>

<gadget id="calculator">27 / 9</gadget>
<output>3</output>

<gadget id="calculator">4 * 3</gadget>
<output>12</output>

<gadget id="calculator">12 - 4</gadget>
<output>8</output>

<result>8</result>