Ports A and B are 125 kilometers apart. It took a passenger ship 5.5 hours to sail from port A to port B. When it returned to port A, it sailed against the current, and it took an extra 1.5 hours. What is the average speed of this passenger ship going back and forth between ports A and B?
甲乙两个港口相距125千米，一艘客轮从甲港口驶往乙港口用了5.5小时，返回甲港口时是逆水行驶，多用了1.5小时，这艘客轮往返甲乙两港口的平均速度是多少？
<gadget id="calculator">125 * 2</gadget>
<output>250</output>

<gadget id="calculator">5.5 + 1.5 + 5.5</gadget>
<output>12.5</output>

<gadget id="calculator">250 / 12.5</gadget>
<output>20</output>

<result>20</result>