Port A and Port B are 120 kilometers apart. It took a ship 3.5 hours to sail from Port A to Port B. It took 1 hour longer to return because of the headwind. How many kilometers per hour does the ship travel on average?
甲、乙两港相距120千米，一艘轮船从甲港开往乙港用了3.5小时，返回时因为逆水比去时多用了1小时．这艘轮船往返平均每小时行多少千米？
<gadget id="calculator">120 * 2</gadget>
<output>240</output>

<gadget id="calculator">3.5 + 1 + 3.5</gadget>
<output>8</output>

<gadget id="calculator">240 / 8</gadget>
<output>30</output>

<result>30</result>