Soybean went to climb the mountain on the weekend. He went up the mountain at a speed of 3 kilometers per hour. After reaching the top of the mountain, he immediately went down the mountain along the same road. The speed of going down the mountain was 6 kilometers per hour. What is the average speed of Xiao Ming going up and down the mountain?
大豆周末去爬山，他上山的速度是每小时3千米，到达山顶后立即沿原路下山，下山的速度是每小时6千米，小明上、下山的平均速度是多少？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">(1/2) + (1/6)</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">2 / (2/3)</gadget>
<output>3</output>

<result>3</result>