2007 AMC 12B Problem 10

Problem 10 of 25EasierAlgebra

Some boys and girls are having a car wash to raise money for a class trip to China. Initially

40\%

of the group are girls. Shortly thereafter two girls leave and two boys arrive, and then

30\%

of the group are girls. How many girls were initially in the group?

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Solution

Since two girls leave while two boys arrive, the total group size is unchanged. The drop from

40\%

30\%

girls corresponds to the two girls who left. So those two girls are

10\%

of the group, meaning the group has

20

people. The initial number of girls was

40\%

20,

8.

Thus, the correct answer is C.