AMC10 2017 B

AMC10 2017 B · Q2

AMC10 2017 B · Q2. It mainly tests Rates (speed), Arithmetic misc.

Sofia ran 5 laps around the 400-meter track at her school. For each lap, she ran the first 100 meters at an average speed of 4 meters per second and the remaining 300 meters at an average speed of 5 meters per second. How much time did Sofia take running the 5 laps?

索菲亚在学校400米跑道上跑了5圈。每圈她前100米以平均速度4米/秒跑，其余300米以平均速度5米/秒跑。索菲亚跑5圈总共用了多少时间？

(A) 5 minutes and 35 seconds 5 分 35 秒

(B) 6 minutes and 40 seconds 6 分 40 秒

(D) 7 minutes and 25 seconds 7 分 25 秒

(E) 8 minutes and 10 seconds 8 分 10 秒

Answer

Correct choice: (C)

正确答案：（C）

Solution

Each lap took Sofia $\frac{100 \text{ m}}{4 \text{ m/s}} + \frac{300 \text{ m}}{5 \text{ m/s}} = 85$ seconds, so 5 laps took her $5 \cdot 85 = 425$ seconds, which is 7 minutes and 5 seconds.

每圈索菲亚用了$\frac{100 \text{ m}}{4 \text{ m/s}} + \frac{300 \text{ m}}{5 \text{ m/s}} = 85$秒，所以5圈用了$5 \cdot 85 = 425$秒，即7分5秒。

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