person goes up the mountain at a speed of 3 kilometers per hour, returns along the original road, and goes down the mountain at a speed of 6 kilometers per hour. What is the average speed of going up and down the mountain in kilometers per hour?
一个人上山的时速是3千米，沿原路返回，下山时速是6千米，上下山平均时速是多少千米．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

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

<gadget id="calculator">(1/3) + (1/6)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">2 / (1/2)</gadget>
<output>4</output>

<result>4</result>