A truck travels from A to B at 60 kilometers per hour, and then returns along the original road at 30 kilometers per hour. Find the average speed of the truck going back and forth.
一辆货车，从A地到B地，每小时60千米，然后再沿原路返回，每小时30千米，求这辆货车往返的平均速度．
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/60) + (1/30)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">2 / (1/20)</gadget>
<output>40</output>

<result>40</result>