求助GWD的一道题

stevenckguo

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


为什么我做的是C呢？？

rchgogogo

应该选D吧。。。