The speed of the car going up the mountain is 30 kilometers per hour, and the speed of going down the mountain is 60 kilometers per hour (returning on the same road). What is the average speed of the car going up and down the mountain in kilometers per hour?
汽车上山的速度是每小时30千米，下山的速度是每小时60千米(按原路返回)，汽车上、下山的平均速度是每小时多少千米．
<gadget id="calculator">1 * 2</gadget>
<output>2</output>

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

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

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

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

<result>40</result>