The speed of going up the mountain is 2.4 km/h, and the speed of going down the mountain by the original road is 3.6 km/h. Find the average speed up and down the hill.
上山的速度为2.4千米/小时，按原路下山的速度为3.6千米/小时．求上下山的平均速度．
<gadget id="calculator">1 / 2.4</gadget>
<output>0.416667</output>

<gadget id="calculator">1 / 3.6</gadget>
<output>0.277778</output>

<gadget id="calculator">0.416667 + 0.277778</gadget>
<output>0.694445</output>

<gadget id="calculator">2 / 0.694445</gadget>
<output>2.879998</output>

<result>2.879998</result>