gwd5-17 求助呀，看似简单我咋就做不出来呢

froglegdiva

Q17:

    

A driver completed the first 20 miles of a 40-mile trip at
an average speed of 50 miles per hour. 
At what average speed must the driver complete the remaining 20 miles to
achieve an average speed of 60 miles per hour for the entire 40-mile trip?  (Assume that the driver did not make any
stops during the 40-mile trip.)

    

 

    

A.    
65 mph

    

B.    
68 mph

    

C.    
70 mph

    

D.    
75 mph

    

E.     
80 mph

    

Answer

为啥不是C呢？谢谢大家指点迷津

[此贴子已经被作者于2009/9/22 23:31:02编辑过]

weinney

前20米的平均速度50m/h,整个40米的平均速度60m/h, 问后20米的平均速度是多少。

设x

20/50+20/x=40/60

ps.题目有点tricky,需要仔细读题