2010 AMC 8 Problem 12

Of the 500 balls in a large bag, 80\% are red and the rest are blue. How many of the red balls must be removed from the bag so that 75\% of the remaining balls are red?

\textbf{(A)}\ 25 \qquad\textbf{(B)}\ 50 \qquad\textbf{(C)}\ 75 \qquad\textbf{(D)}\ 100\qquad\textbf{(E)}\ 150

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