During the Ching Ming Festival holiday, a school organized a mountain climbing activity for fourth-grade students. When they went up the mountain, they walked 30 meters per minute for 40 minutes. When they went down the mountain, they went back the same way and walked 60 meters per minute. Find their average speed up and down the hill.
清明节假期，某校组织四年级学生进行了一次爬山活动，上山时他们每分钟走30米，走了40分钟，下山时，他们原路返回，每分钟走60米．求他们上下山的平均速度．
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/30) + (1/60)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">2 / (1/20)</gadget>
<output>40</output>

<result>40</result>