AMC 12 2007b Problem 10

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?

$\mathrm {(A)} 4 \mathrm {(B)} 6 \mathrm {(C)} 8 \mathrm {(D)} 10 \mathrm {(E)} 12$