Port A and Port B are 140 kilometers apart. It takes 5.5 hours for a ship to travel from Port A to Port B. When it returns, it takes 1 hour less than the time when it returns. What is the average speed of the ship in kilometers per hour? ．
甲、乙两港相距140千米，一艘轮船从甲港开往乙港用5.5小时，返回时因为顺水，比去时少用1小时，这艘轮船往返的平均速度是多少千米/小时．
<gadget id="calculator">5.5 - 1</gadget>
<output>4.5</output>

<gadget id="calculator">5.5 + 4.5</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>