1997 Cayley Problem 20

On a circle, fifteen points A_1, A_2, A_3, \ldots, A_{15} are equally spaced. What is the size of angle A_1A_3A_7?

\textbf{(A)}\ 96^\circ\quad \textbf{(B)}\ 100^\circ\quad \textbf{(C)}\ 104^\circ\quad \textbf{(D)}\ 108^\circ\quad \textbf{(E)}\ 120^\circ

If there are no answer choices shown, enter a numerical answer.

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