Uncle Li went up the mountain to collect herbs. It took him 18 minutes to walk 900 meters to the top of the mountain when he went up the mountain. Ask Uncle Li what is the average speed of going up and down the mountain?
李大伯上山采药，上山时他走了900米到达山顶用了18分钟，下山时他按原路返回，只用了12分钟．问李大伯上下山的平均速度是多少？
<gadget id="calculator">900 * 2</gadget>
<output>1_800</output>

<gadget id="calculator">18 + 12</gadget>
<output>30</output>

<gadget id="calculator">1_800 / 30</gadget>
<output>60</output>

<result>60</result>