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How much time did it take a certain car to travel 400 kilometers?
(1) The car traveled the first 200 kilometers in 2.5 hours.
(2) If the car's average speed had been 20 kilometers per hour greater than it was, it would have traveled the 400 kilometers in 1 hour less time than it did.
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【答案】B
【思路】距離 = 速率 x 時間
(運算時注意單位,AT真題中數據常會時間與分鐘混用,干擾判斷)
(1) 顯然不充分,沒給後200km路程的相關資訊。
(2) 設average speed為v(km/hr),total travel time為t(hr)
則由題幹知 vt = 400
由(2)知 (v + 20)(t - 1) = 400
第一式代入第二式解聯立方程:
(這邉我貼不出原來完整圖檔,簡單寫出有問題處)
vt-v+20t-20=400==> 代入vt=400後,到最後會得出一個方程式:t^2-t+20=0
(t-5)(t+4)=0 ===>這個部分有錯喔,有沒有高手會解?
可解出 t = 5小時 (-4不合)
這題答案我本來選E的