Add 18 to the denominator of (2/9), how much should be added to the numerator to keep the size of the fraction unchanged.
(2/9)的分母加上18，要使分数的大小不变，分子应加上多少．
<gadget id="calculator">9 + 18</gadget>
<output>27</output>

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

<gadget id="calculator">2 * 3</gadget>
<output>6</output>

<gadget id="calculator">6 - 2</gadget>
<output>4</output>

<result>4</result>