The distance between A and B ports is 180 kilometers. It took a ship 5.5 hours to sail from port A to port B, and it took 1 hour longer to return because of headwinds. What is the average round-trip speed of the ship in kilometers per hour?
甲乙两港相距180千米．一艘轮船从甲港驶往乙港用了5.5小时，返回时因为逆水比去时多用了1小时，则这艘船的往返平均速度为多少千米/时．
<gadget id="calculator">5.5 + 5.5 + 1</gadget>
<output>12</output>

<gadget id="calculator">2 / 12</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">180 * (1/6)</gadget>
<output>30</output>

<result>30</result>