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
是D
我的答案是70
思維如下 請問哪裡錯了?
設第二段路程速率為S 由(第一段速率+第二段速率)/2 => (50+S)/ 2=60,S=70
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