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：

其他年份的第 2 题