(c) For solid cylinder, $\theta = 30{^\circ},\ K^{2} = \frac{1}{2}R^{2}$
For hollow cylinder, $\theta = ?,\ K^{2} = R^{2}$
Using we find,
$\frac{\left( 1 + \frac{1}{2} \right)}{\sin{30{^\circ}}} = \frac{1 + 1}{\sin\theta}$
$\therefore\ \sin\theta = \frac{2}{3} = 0.6667$
$\theta = 42{^\circ}$