(c) $E = \frac{1}{2}m\omega^{2}r^{2}\ \ ie,\ \ \ E \propto (\omega r)^{2}$
or $\left( \omega_{1}r_{1} \right)^{2} = \left( \omega_{2}r_{2} \right)^{2}\ \ \ or\ \ \ \omega_{1}r_{1} = \omega_{2}r_{2}$
or $10 \times 8 = \omega \times 5\ \ or\ \ \ \omega = 16\ units$