person goes up the mountain at a speed of 3 kilometers per hour, and then returns to the same place at a speed of 6 kilometers per hour. What is the average speed of this person for one round trip?
某人以每小时3千米的速度上山，然后又以每小时6千米的速度返回原地，此人往返一次的平均速度是多少．
<result>4</result>