1955 AHSME Problem 3

If each number in a set of ten numbers is increased by 20, the arithmetic mean (average) of the ten numbers:

\begin{array}{l} \textbf{(A)}\ \text{remains the same}\qquad\textbf{(B)}\ \text{is increased by 20}\qquad\textbf{(C)}\ \text{is increased by 200}\\ \textbf{(D)}\ \text{is increased by 10}\qquad\textbf{(E)}\ \text{is increased by 2} \end{array}

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