If a person travels from point A to point B at an average speed of 3 m/s, and travels 7 m/s when returning the same way, what is the average speed of this person in one round trip?
某人从A地到B地平均速度为3米/秒，按原路返回时每秒行7米，那么此人一个来回的平均速度是多少米/秒．
<result>4.2</result>