Port A and port B are 280 kilometers apart. It takes a ship 9 hours to sail from port A to port B. It takes 2 hours longer to return because of headwinds. What is the average speed of the ship going back and forth?
甲、乙两港相距280千米，一艘船从甲港驶往乙港用了9小时，返回时因为逆水多用了2小时，求这艘船往返的平均速度是多少？
<gadget id="calculator">280 * 2</gadget>
<output>560</output>

<gadget id="calculator">9 + 2 + 9</gadget>
<output>20</output>

<gadget id="calculator">560 / 20</gadget>
<output>28</output>

<result>28</result>