Multiply the denominator of (20/29) by 2, how much should be added to the numerator to keep the size of the fraction constant?
(20/29)的分母乘以2，要使分数的大小不变，分子应加上多少？
<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">40 - 20</gadget>
<output>20</output>

<result>20</result>