AMC10 2003 A

AMC10 2003 A · Q4

AMC10 2003 A · Q4. It mainly tests Rates (speed), Arithmetic misc.

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?

Mary上坡从家走到学校1 km需要30分钟，但从学校沿同一路线走回家只需10分钟。她的往返平均速度是多少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

Mary walks a total of 2 km in 40 minutes. Because 40 minutes is $\frac{2}{3}$ hr, her average speed, in km/hr, is $\frac{2}{\frac{2}{3}} = 3$.

Mary总共走2 km，用时40分钟。因为40分钟是$\frac{2}{3}$小时，她的平均速度为$\frac{2}{\frac{2}{3}} = 3$ km/hr。

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