Xiao Ming's speed up the mountain is 3 kilometers per hour, and his speed down the mountain is 6 kilometers per hour. What is the average speed of Xiao Ming going up the mountain and then going down the mountain in kilometers per hour?
小明上山的速度是每小时3千米，下山的速度是每小时6千米．小明上山后再下山的平均速度是每小时多少千米．
<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>