$$16\sin 20^\circ \,.\,\sin 40^\circ \,.\,\sin 80^\circ $$

$$ = 4\sin 60^\circ $$ {$$\because$$ $$4\sin \theta \,.\,\sin (60^\circ - \theta )\,.\,\sin (60^\circ + \theta ) = \sin 3\theta $$}

$$ = 2\sqrt 3 $$