Some boys and girls are having a car wash to raise money for a class trip to China. Initially of the group are girls. Shortly thereafter two girls leave and two boys arrive, and then of the group are girls. How many girls were initially in the group?
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Problem © Mathematical Association of America (MAA), American Mathematics Competitions. Reproduced for non-commercial educational use. Solution and commentary are original to this site. Also appeared as 2007 12B #10.