gwd 5 17

alexgong

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

pennmou

因该用时间来算。
前20mile 用时20除以50  时间为五分之二小时    如果全程均速为60mile per hour, 那么总共用时 40 除以60 等于3分之2 小时。 用3分之2 减去 5分之2等于15分之4， 就是后20mile允许的用时。然后用后面的20mile 除以时,15分之4 就是后半段的时速，等于75mile per hour.

alexgong

謝謝你 非常受用 隔幾個月再看書的我 就要考了呢 謝謝你的及時雨