2007 AMC 12B 第 13 题
先试着解答 2007 AMC 12B 第 13 题，然后核对你的答案与精心整理的解答，解答来自 LIVE by Po-Shen Loh。你也可以参加完整限时模拟考试、查看全部 2007 AMC 12B 解答，或核对答案。
所有题目均经美国数学协会（MAA）官方合法授权使用。
13.
一个交通灯不断重复以下周期：绿灯 秒，然后黄灯 秒，然后红灯 秒。Leah 随机选择一个三秒钟的时间段观察交通灯。在她观察期间灯变色的概率是多少？
A traffic light runs repeatedly through the following cycle: green for seconds, then yellow for seconds, and then red for seconds. Leah picks a random three-second time interval to watch the light. What is the probability that the color changes while she is watching?
答案：D
小提示：
一个完整周期为 秒。
One full cycle lasts seconds
大提示：
只有当她的观察区间在某个变色时刻前 秒内开始时，她才会看到变色。
A change is seen only if her interval begins within seconds before one of the three color switches
解答：
周期长为 秒，每周期有三次变色。
Leah 看到变色当且仅当她的三秒观察区间在某次变色前的 秒内开始。共有 秒的有利开始时间，在 秒周期中概率为
所以正确答案是 D。
The cycle length is seconds, with three color changes per cycle.
Leah sees a change exactly when her three-second interval starts within the seconds before a switch. That gives favorable seconds out of a probability of
Thus, the correct answer is D.
其他年份的第 13 题
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