(c) $y = 0.2\sin{(10\pi\ t + 1.5\pi)}\cos{(10\pi\ t + 1.5\pi)}$
$= 0.1\sin{2(10\pi t + 1.5\pi)}\ \ \ \lbrack\because\sin{2A} = 2\sin{A\cos A}\rbrack$
$= 0.1\sin(20\pi\ t + 3.0\pi)$
$\therefore Time\ period,\ T = \frac{2\pi}{\omega} = \frac{2\pi}{20\pi} = \frac{1}{10} = 0.1\ sec$