20. Evaluate $256 \sin 10^{\circ} \sin 30^{\circ} \sin 50^{\circ} \sin 70^{\circ}$.
$$
\begin{array}{l}
256 \sin 10^{\circ} \sin 30^{\circ} \sin 50^{\circ} \sin 70^{\circ} \\
=\quad \frac{2 \times 256 \sin 10^{\circ} \cos 10^{\circ} \sin 30^{\circ} \cos 40^{\circ} \cos 20^{\circ}}{2 \cos 10^{\circ}} \\
=\quad \frac{128 \sin 20^{\circ} \cos 20^{\circ} \cos 40^{\circ}}{2 \cos 10^{\circ}} \\
=\quad \frac{64 \sin 40^{\circ} \cos 40^{\circ}}{2 \cos 10^{\circ}} \\
=\quad \frac{32 \sin 80^{\circ}}{2 \cos 10^{\circ}} \\
=\quad 16 .
\end{array}
$$
20. Answer: 16