2007 AMC 10B Problem 14

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?

\textbf{(A) } 4 \qquad\textbf{(B) } 6 \qquad\textbf{(C) } 8 \qquad\textbf{(D) } 10 \qquad\textbf{(E) } 12

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.