Xiao Li's speed is 3 kilometers per hour when he goes up the mountain, and 6 kilometers per hour when he goes down the mountain. What is his average speed in kilometers when he goes up and down the mountain?
小李上山时速度是每小时3千米，下山时速度是每小时6千米，他上、下山的平均速度是多少千米．
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<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>