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题库 / Prep2007E1-PS-153
A boat traveled upstream adistance of 90 miles at an average speed of (v - 3) miles per hour and thentraveled the same distance downstream at an average speed of (v + 3) miles perhour. If the trip upstream took half anhour longer than the trip downstream, how many hours did it take the boat totravel downstream?
2.5
2.4
2.3
2.2
2.1
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90=r(t+0.5)=(r+6)t 得出r=12t。把r=12t 代入上式，得：
2t^2+t-15=0 解得t1=2.5,t2=-3。
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