2002 AMC 12A 第 11 题
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所有题目均经美国数学协会（MAA）官方合法授权使用。
11.
Earl E. Bird 先生每天上午 整离家上班。当他的平均速度为每小时 英里时，会晚到三分钟；当平均速度为每小时 英里时，会早到三分钟。Bird 先生应以多少英里每小时的平均速度行驶，才能准时到达工作地点？
Mr. Earl E. Bird leaves his house for work at exactly A.M. every morning. When he averages miles per hour, he arrives at his workplace three minutes late. When he averages miles per hour, he arrives three minutes early. At what average speed, in miles per hour, should Mr. Bird drive to arrive at his workplace precisely on time?
小提示：
设准时所需行驶时间为 小时；三分钟是 小时。
Let be the on-time travel time in hours; three minutes is hours
大提示：
列方程 ，求出 ，再求路程。
Set solve for then find the distance
解答：
设准时所需时间为 小时。三分钟是 小时，所以 。
得 ，所以 。路程为 英里，所需速度为 英里每小时。
因此，正确答案是 B。
Let be the time in hours to arrive on time. Since three minutes is hours,
This gives so The distance is miles, and the required speed is miles per hour.
Thus, the correct answer is B.
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