1973 AHSME Problem 33

When one ounce of water is added to a mixture of acid and water, the new mixture is 20\% acid. When one ounce of acid is added to the new mixture, the result is 33\frac13\% acid. The percentage of acid in the original mixture is

\textbf{(A)}\ 22\% \qquad \textbf{(B)}\ 24\% \qquad \textbf{(C)}\ 25\% \qquad \textbf{(D)}\ 30\% \qquad \textbf{(E)}\ 33\frac13 \%

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