prep的题目

Susan drove at an average speed of 30 miles per hour for the first 30 miles for a trip, and then at an average speed of 60 miles per hour for the remaining 30 miles of the trip . If she made no stops during the trip, what was Susan's average speed, in miles per hour, for the entire trip.

a.35

b.40

c.45

d.50

e.55   (b)

这题很简单,但是我却写不出来,我的答案是45

不知道为什么是40,求教了

第一段時間為30/30=1h...第二段時間為30/60=0.5h

總距離為60..總時間為1.5...應該算得出來吧?

谢谢两位,我看错题目了