Ports A and B are 120 kilometers apart. A freighter travels 60 kilometers per hour from upstream port A to downstream port B; and then returns along the original road at 30 kilometers per hour. What is the average speed in kilometers per hour.
甲、乙两港口相距120千米，一艘货轮从上游甲港口到下游的乙港口，每小时行60千米；然后再沿原路返回，每小时行30千米，则这艘货轮往返的平均速度是每小时多少千米．
<gadget id="calculator">120 / 60</gadget>
<output>2</output>

<gadget id="calculator">120 / 30</gadget>
<output>4</output>

<gadget id="calculator">2 + 4</gadget>
<output>6</output>

<gadget id="calculator">2 / 6</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">120 * (1/3)</gadget>
<output>40</output>

<result>40</result>