After the denominator of (9/16) is doubled, how much should be added to the numerator to keep the size of the fraction unchanged.
(9/16)的分母扩大2倍后，要使分数的大小不变，分子应加上多少．
<gadget id="calculator">9 * 2</gadget>
<output>18</output>

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

<result>9</result>