2011 AMC 10A 第 8 题

Question

去年夏天，Town Lake 上的鸟中

30 \%

是鹅，

25 \%

是天鹅，

10 \%

是鹭，

35 \%

是鸭。不是天鹅的鸟中，百分之多少是鹅？

Last summer

30 \%

of the birds living on Town Lake were geese,

25 \%

were swans,

10 \%

were herons, and

35 \%

were ducks. What percent of the birds that were not swans were geese?

20

30

40

50

60

用不是天鹅的百分比作分母。

Use the percent that are not swans as the denominator

所求百分比为

\frac{30}{100-25}

。

The desired percent is

\frac{30}{100-25}

不妨设共有

100

只鸟，则不是天鹅的有

75

只。所求百分比为

\dfrac{30}{75} \cdot 100 = 40 \%\text{。}

所以正确答案是 C。

WLOG, let there be

100

birds. Then

75

birds are not swans. The desired percentage is then

\dfrac{30}{75} \cdot 100 = 40 \%.

Thus, C is the correct answer.

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