Port A and port B are 140 kilometers apart. It took a ship 4.5 hours to sail from port A to port B. When returning, it took 1 hour longer than when going back because of the headwind. Find the average round trip speed of the ship.
甲乙两港相距140千米，一艘轮船从甲港驶向乙港用了4.5小时，返回时因为逆水比去时多用1小时．求这艘轮船往返的平均速度．
<gadget id="calculator">140 * 2</gadget>
<output>280</output>

<gadget id="calculator">4.5 + 1 + 4.5</gadget>
<output>10</output>

<gadget id="calculator">280 / 10</gadget>
<output>28</output>

<result>28</result>