There is a mountain. The distance from the foot of the mountain to the top is 6 kilometers. It is obvious that 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. Then the average speed of going up and down the mountain is 1 hour per hour. How many kilometers?
有一座山，山脚到山顶的距离是6千米，明明上山时的速度是每小时走2千米，下山时的速度是每小时走6千米，那么他上、下山的平均速度是每小时多少千米？
<gadget id="calculator">6 * 2</gadget>
<output>12</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

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

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">12 / 4</gadget>
<output>3</output>

<result>3</result>