2018 AMC 12B 第 2 题

Question

Sam 在

90

分钟内开了

96

英里。他前

30

分钟的平均速度为

60

mph，第二个

30

分钟的平均速度为

65

mph。他最后

30

分钟的平均速度是多少 mph？

Sam drove

96

miles in

90

minutes. His average speed during the first

30

minutes was

60

mph (miles per hour), and his average speed during the second

30

minutes was

65

mph. What was his average speed, in mph, during the last

30

minutes?

64

65

66

67

68

答案：D

知识点：路程、速度与时间

难度评级：1080

小提示：

先求前两个

30

分钟各行驶了多少距离。

Find the distance covered in each of the first two

30

-minute segments

大提示：

剩余距离在

\tfrac12

小时内行驶；用距离除以时间得到速度。

The remaining distance is covered in

\tfrac12

hour; divide to get the speed

解答：

前

30

分钟 Sam 行驶

60\cdot\tfrac12=30

英里，接着又行驶

65\cdot\tfrac12=32.5

英里。

最后

30

分钟行驶

96-30-32.5=33.5

英里，所以速度为

\dfrac{33.5}{\frac{1}{2}}=67\text{ 英里/小时}\text{。}

所以正确答案是 D。

In the first

30

minutes Sam covered

60\cdot\tfrac12=30

miles, and in the second he covered

65\cdot\tfrac12=32.5

miles.

The last

30

minutes covered

96-30-32.5=33.5

miles, so the speed was

\dfrac{33.5}{\frac{1}{2}}=67\text{ mph}.

Thus, the correct answer is D.

2018 AMC 12B 第 2 题

答案：D

难度评级：1080

小提示：

大提示：

解答：

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