Xiao Ming climbs a mountain with a speed of 4 km/h when going up the mountain and 6 km/h when going down the mountain. What is his average speed of climbing the mountain in km/h?
小明爬一座山，上山时速度为4千米/时，下山时速度为6千米/时，他爬山的平均速度是多少千米/时．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/4) + (1/6)</gadget>
<output>5/12 = around 0.416667</output>

<gadget id="calculator">2 / (5/12)</gadget>
<output>24/5 = around 4.8</output>

<result>24/5 = around 4.8</result>