AMC12 2018 B

AMC12 2018 B · Q2

AMC12 2018 B · Q2. It mainly tests Rates (speed), Arithmetic misc.

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?

Sam在90分钟内开车96英里。他前30分钟的平均速度是60 mph（英里每小时），第二30分钟的平均速度是65 mph。最后30分钟的平均速度是多少mph？

(A) 64 64

(B) 65 65

(D) 67 67

(E) 68 68

Answer

Correct choice: (D)

正确答案：（D）

Solution

First 30 min ($\frac{1}{2}$ h): $60 \times 0.5 = 30$ miles.\nSecond 30 min: $65 \times 0.5 = 32.5$ miles.\nLast 30 min: $96 - 30 - 32.5 = 33.5$ miles in $0.5$ h, so speed $= 33.5 / 0.5 = 67$ mph.

前30分钟（$\frac{1}{2}$小时）：$60 \times 0.5 = 30$英里。第二30分钟：$65 \times 0.5 = 32.5$英里。最后30分钟：$96 - 30 - 32.5 = 33.5$英里，用0.5小时，所以速度$= 33.5 / 0.5 = 67$ mph。

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