AHSME 1952 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: $\textbf{(A)}\ 75\% \textbf{(B)}\ 74\% \textbf{(C)}\ 72\% \textbf{(D)}\ 77\% \textbf{(E)\ }\text{none of these}$