2010 AMC 10B 第 2 题

Question

Makayla 在

9

小时工作日中参加了两场会议。第一场会议用了

45

分钟，第二场会议用时是第一场的两倍。她工作日的百分之多少用于参加会议？

Makayla attended two meetings during her

9

-hour work day. The first meeting took

45

minutes and the second meeting took twice as long. What percent of her work day was spent attending meetings?

15

20

25

30

35

把会议时间换成相同单位。

Convert the meeting times to the same units

两场会议共

45+90

分钟，而工作日为

9

小时。

The meetings last

45+90

minutes out of

9

hours

45

分钟等于

\dfrac{45}{60} = \dfrac{3}{4}

小时。第二场会议用时为

2 \cdot \dfrac{3}{4} = \dfrac{3}{2}

小时，所以两场会议共用

\dfrac{3}{4} + \dfrac{3}{2} = \dfrac{9}{4}

小时。这占整个工作日的

100 \% \cdot \dfrac{\frac{9}{4}}{9} = 100 \% \cdot \dfrac{1}{4} = 25 \%\text{。}

所以正确答案是 C。

Note that

45

minutes is

\dfrac{45}{60} = \dfrac{3}{4}

hours. The second meeting is then

2 \cdot \dfrac{3}{4} = \dfrac{3}{2}

hours. Both meetings take a total of

\dfrac{3}{4} + \dfrac{3}{2} = \dfrac{9}{4}

hours then. The percent of Makayla’s work day spent attending meetings is

100 \% \cdot \dfrac{\frac{9}{4}}{9} = 100 \% \cdot \dfrac{1}{4} = 25 \%.

Thus, C is the correct answer.

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