Port A and Port B are 156 kilometers apart. It took a ship 3.5 hours to sail from Port A to Port B. It took 1 hour longer to return because of headwinds. What is the average speed of the ship going back and forth?
甲、乙两港相距156千米，一艘轮船从甲港驶往乙港用了3.5小时，返回时因为逆水比去时多用了1小时，求这艘轮船往返的平均速度？
<gadget id="calculator">156 * 2</gadget>
<output>312</output>

<gadget id="calculator">3.5 + 1 + 3.5</gadget>
<output>8</output>

<gadget id="calculator">312 / 8</gadget>
<output>39</output>

<result>39</result>