Rotational Motion

A thin circular disc of mass \(\mathrm{M}\) and radius \(\mathrm{R}\) is rotating in a horizontal plane about an axis passing through its centre and perpendicular to its plane with angular velocity \(\omega\). If another disc of same dimensions but of mass \(\mathrm{M} / 2\) is placed gently on the first disc co-axially, then the new angular velocity of the system is :