1952 AHSME Problem 2

Two high school classes took the same test. One class of 20 students made an average grade of 80\%; the other class of 30 students made an average grade of 70\%. The average grade for all students in both classes is:

\textbf{(A)}\ 75\%\qquad \textbf{(B)}\ 74\%\qquad \textbf{(C)}\ 72\%\qquad \textbf{(D)}\ 77\%\qquad \textbf{(E)\ }\text{none of these}

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