Ding Ding goes up the mountain at a speed of 60 meters per minute, reaches the top of the mountain in 15 minutes, then goes down the mountain on the same road, and returns to the starting point in 10 minutes. What is the average speed of Ding Ding going up and down the mountain once?
小丁丁上山的速度为60米/分钟，15分钟到达山顶，然后按原路下山，10分钟回到出发地．小丁丁上、下山往返一次的平均速度是多少？
<gadget id="calculator">60 * 15 * 2</gadget>
<output>1_800</output>

<gadget id="calculator">15 + 10</gadget>
<output>25</output>

<gadget id="calculator">1_800 / 25</gadget>
<output>72</output>

<result>72</result>