The distance between A and B is 140 km. It took a ship 4.5 hours to sail from port A to port B, and it took 5.5 hours to return because of headwinds. What is the average speed of the ship in kilometers per hour?
甲、乙两港相距140千米．一艘轮船从甲港驶往乙港用了4.5小时，返回时因为逆水用了5.5小时．这艘轮船往返的平均速度是每小时多少千米．
<gadget id="calculator">4.5 + 5.5</gadget>
<output>10</output>

<gadget id="calculator">2 / 10</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">140 * (1/5)</gadget>
<output>28</output>

<result>28</result>