2007 AMC 10B 第 3 题

Question

一名大学生周末开小型车回家

120

英里，平均每加仑行驶

30

英里。返程时他开父母的 SUV，平均每加仑只行驶

20

英里。往返全程平均每加仑行驶多少英里？

A college student drove his compact car

120

miles home for the weekend and averaged

30

miles per gallon. On the return trip the student drove his parents’ SUV and averaged only

20

miles per gallon. What was the average gas mileage, in miles per gallon, for the round trip?

22

24

25

26

28

平均燃油效率是总距离除以总耗油量，不是两个数值的普通平均。

Average mileage is total distance divided by total gallons, not the average of the two rates

先求两段路各用了多少加仑，再用

240

除以总加仑数。

Find the gallons used each way, then divide

240

by their sum

回家用了

\frac{120}{30}=4

加仑，返程用了

\frac{120}{20}=6

加仑，共用

10

加仑，行驶

240

英里。

平均为

\frac{240}{10} = 24

英里/加仑。

所以正确答案是 B。

The student used

\frac{120}{30}=4

gallons driving home and

\frac{120}{20}=6

gallons returning, for

10

gallons over

240

miles.

The average is

\frac{240}{10} = 24

miles per gallon.

Thus, the correct answer is B.

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