Add 4 to the numerator of (2/5), how much should be added to the denominator to keep the size of the original fraction unchanged.
(2/5)的分子加上4，要使原分数的大小不变，分母应该加上多少．
<gadget id="calculator">5 + 4</gadget>
<output>9</output>

<gadget id="calculator">9 / 5</gadget>
<output>9/5 = around 1.8</output>

<gadget id="calculator">2 * (9/5)</gadget>
<output>18/5 = around 3.6</output>

<gadget id="calculator">(18/5) - 2</gadget>
<output>8/5 = around 1.6</output>

<result>8/5 = around 1.6</result>