A mountain road has a total length of 300 kilometers. Xiao Ming’s speed is 10 kilometers per hour when he goes up the mountain, and 15 kilometers per hour when he goes down the mountain. What is his average speed up and down the mountain in km/h? (Hint: first calculate the total distance up and down the mountain, then calculate the total time, and finally use the formula to calculate.)
一条山路总长300千米，小明上山时的速度是10千米/小时，下山时的速度是15千米/小时．他上山下山的平均速度是多少千米/小时？(提示：先求上山下山的总路程，再求总时间，最后用公式计算．)
<gadget id="calculator">300 + 300</gadget>
<output>600</output>

<gadget id="calculator">300 / 10</gadget>
<output>30</output>

<gadget id="calculator">300 / 15</gadget>
<output>20</output>

<gadget id="calculator">30 + 20</gadget>
<output>50</output>

<gadget id="calculator">600 / 50</gadget>
<output>12</output>

<result>12</result>