AHSME 1962 Problem 17

If $a = \log_8 225$ and $b = \log_2 15$, then $a$, in terms of $b$, is: $\textbf{(A)}\ \frac{b}{2} \textbf{(B)}\ \frac{2b}{3} \textbf{(C)}\ b \textbf{(D)}\ \frac{3b}{2} \textbf{(E)}\ 2b$