Xiao Ming walks a 12-kilometer mountain road and returns along the original road. The speed when going up the mountain is 2 kilometers per hour, and the speed when going down the mountain is 6 kilometers per hour. What is the average speed of Xiao Ming going up and down the mountain?
小明走一条12千米的山路，又沿原路返回，上山的时候速度是每小时2千米，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">12 * 2</gadget>
<output>24</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">12 / 6</gadget>
<output>2</output>

<gadget id="calculator">6 + 2</gadget>
<output>8</output>

<gadget id="calculator">24 / 8</gadget>
<output>3</output>

<result>3</result>