Port A and port B are 120 kilometers apart. It takes 5.5 hours for a ship to sail from port A to port B. When returning, it takes 1 hour less than when going back. Find the average speed of the ship going back and forth.
甲乙两港相距120千米，一艘轮船从甲港驶往乙港用了5.5小时，返回时因为顺水比去时少用了1小时，求这艘轮船往返的平均速度．
<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">120 * (1/5)</gadget>
<output>24</output>

<result>24</result>