2007 AMC 10B Problem 14

Problem 14 of 25IntermediateAlgebra

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

Two girls leave and two boys arrive, so the group size is unchanged. The two girls who left therefore represent

40\%-30\%=10\%

of the group. Thus the group has

2\div 0.10=20

people, and the original number of girls was

40\%

20,

8.

Thus, the correct answer is C.