Rich and Ben ate an entire chocolate cake. The ratio of the amount eaten by Rich to the amount eaten by Ben is $3: 1$. What percentage of the cake did Ben eat?
(A) $66 \%$
(B) $50 \%$
(C) $75 \%$
(D) $25 \%$
(E) $10 \%$
The ratio of the amount of cake eaten by Rich to the amount of cake eaten by Ben is $3: 1$.

Thus, if the cake was divided into 4 pieces of equal size, then Rich ate 3 pieces and Ben ate 1 piece or Ben ate $\frac{1}{4}$ of the cake.

Converting to a percent, Ben ate $\frac{1}{4} \times 100 \%=0.25 \times 100 \%=25 \%$ of the cake.