Apply conservation of angular momentum.
We have, \(I_1 \omega_1=I_2 \omega_2\)
\(\omega_2=\frac{I_1}{I_2} \omega_1\)
\(\omega_2=\frac{(1 / 2) M R^2}{\left(1 / 2\right) M R^2+(1 / 2)\left(1 / 4\right) M R^2} \omega=\frac{4}{5} \omega\)
$(\left.g=10 \,m / s ^{2}\right)$