1950 AHSME Problem 22

Successive discounts of 10\% and 20\% are equivalent to a single discount of:

\textbf{(A)}\ 30\%\qquad\textbf{(B)}\ 15\%\qquad\textbf{(C)}\ 72\%\qquad\textbf{(D)}\ 28\%\qquad\textbf{(E)}\ \text{None of these}

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