Xiaoye walks an 18-kilometer mountain road and returns up the mountain along the original road at a speed of 3 km/h, and downhill at a speed of 6 km/h. What is the average speed of Xiaoye going up and down the mountain?
小叶走一条18千米的山路，又沿原路返回上山时候的速度是3千米/时，下山的速度是6千米/时，小叶上下山的平均速度是多少？
<gadget id="calculator">18 * 2</gadget>
<output>36</output>

<gadget id="calculator">18 / 3</gadget>
<output>6</output>

<gadget id="calculator">18 / 6</gadget>
<output>3</output>

<gadget id="calculator">6 + 3</gadget>
<output>9</output>

<gadget id="calculator">36 / 9</gadget>
<output>4</output>

<result>4</result>