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2011 AMC 12A Problem 5

Problem 5 of 25EasierAlgebraCounting & Probability

Last summer 30%30\% of the birds living on Town Lake were geese, 25%25\% were swans, 10%10\% were herons, and 35%35\% were ducks. What percent of the birds that were not swans were geese?

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Solution

The birds that are not swans make up 100%−25%=75%100\% - 25\% = 75\% of the total, and geese are 30%30\% of the total. The requested fraction is 3075=25=40%. \dfrac{30}{75} = \dfrac{2}{5} = 40\%. Thus, the correct answer is C.

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Concepts: percentage · conditional probability

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