Prep 數學

[plilo]

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

answer: 40


問這一題還蠻不好意思的
可是我真得不知道我的算法到底哪裡錯了

我的算法是：（30*30+60*30）/60＝45
到底是哪裡出錯了？
請大家幫幫我

[annitatu]

30miles/30mph=1hour
30miles/60mph=0.5hour
(30+30)miles/1.5hour=40mph
速度=哩數/小時
所以用速度乘以哩數算出來就不對了…