Xiao Ming rides uphill at a speed of 10 kilometers per hour, and returns to the original road at a downhill speed of 20 kilometers per hour. Find the average speed of Xiao Ming's uphill and downhill.
小明骑车上坡速度是每小时10千米，原路返回下坡速度是每小时20千米，求小明上、下坡的平均速度．
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/10) + (1/20)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">2 / (3/20)</gadget>
<output>40/3 = around 13.333333</output>

<result>40/3 = around 13.333333</result>