Question

Mary 从家到学校上坡走 $1$ 千米需要 $30$ 分钟，但沿同一路线从学校回家只需 $10$ 分钟。她往返全程的平均速度是多少千米每小时？

It takes Mary $30$ minutes to walk uphill $1$ km from her home to school, but it takes her only $10$ minutes to walk from school to home along the same route. What is her average speed, in km/hr, for the round trip?

$3$

$3.125$

$3.5$

$4$

$4.5$

答案：A

知识点：路程、速度与时间调和平均数

难度评级：1130

解答：

Mary 总共走了 $2$ 千米，用时 $30 + 10 = 40$ 分钟。

因为 $40$ 分钟是 $\dfrac{2}{3}$ 小时，平均速度为 $2 \div \dfrac{2}{3} = 3$ 千米每小时。

所以正确答案是 A。

Mary walks a total of $2$ km in $30 + 10 = 40$ minutes.

Since $40$ minutes is $\dfrac{2}{3}$ hour, her average speed is $2 \div \dfrac{2}{3} = 3$ km/hr.

Thus, the correct answer is A.

2003 AMC 10A 第 4 题

答案：A

难度评级：1130

解答：

其他年份的第 4 题