AHSME 1951 Problem 22

The values of $a$ in the equation: $\log_{10}(a^2 - 15a) = 2$ are: $\textbf{(A)}\ \frac {15\pm\sqrt {233}}{2} \textbf{(B)}\ 20, - 5 \textbf{(C)}\ \frac {15 \pm \sqrt {305}}{2}$ $\textbf{(D)}\ \pm20 \textbf{(E)}\ \text{none of these}$