Xiao Zhang went up the mountain along a 6-kilometer-long mountain road, and then went down the same road. The speed when going up the mountain is 3 kilometers per hour, and the speed when going down the mountain is 6 kilometers per hour. How many kilometers per hour is Xiao Zhang's average speed during the whole process of going up and down the mountain?
小张沿着一条长为6千米的山路上山，又从原路下山．上山时的速度是每小时3千米，下山时的速度是每小时6千米．小张上下山全过程中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 / 3</gadget>
<output>2</output>

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

<gadget id="calculator">2 + 1</gadget>
<output>3</output>

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

<result>4</result>