AMC12 2003 A

AMC12 2003 A · Q4

AMC12 2003 A · Q4. It mainly tests Rates (speed), Arithmetic misc.

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

Anna 从家到学校沿同一路线上坡走 $1$ km 需要 $30$ 分钟，但从学校沿同一路线回家只需要 $10$ 分钟。她往返的平均速度是多少（单位：km/hr）？

(A) 3 3

(B) 3.125 3.125

(D) 4 4

(E) 4.5 4.5

Answer

Correct choice: (A)

正确答案：（A）

Solution

Since she walked $1$ km to school and $1$ km back home, her total distance is $1+1=2$ km. Since she spent $30$ minutes walking to school and $10$ minutes walking back home, her total time is $30+10=40$ minutes = $\frac{40}{60}=\frac{2}{3}$ hours. Therefore her average speed in km/hr is $\frac{2}{\left(\frac{2}{3}\right)}=\boxed{\mathrm{(A)}\ 3}$.

她去学校走了 $1$ km，回家又走了 $1$ km，所以总路程为 $1+1=2$ km。她去学校用时 $30$ 分钟，回家用时 $10$ 分钟，所以总时间为 $30+10=40$ 分钟 = $\frac{40}{60}=\frac{2}{3}$ 小时。因此她的平均速度（km/hr）为 $\frac{2}{\left(\frac{2}{3}\right)}=\boxed{\mathrm{(A)}\ 3}$。

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