A disk of mass M is spinning freely at 6.99 rad/s when a second identical disk, initially not spinning, is dropped onto it so

Question

A disk of mass M is spinning freely at 6.99 rad/s when a second identical disk, initially not spinning, is dropped onto it so that their axes coincide. In a short time the two disks are corotating. 
(a) What is the angular speed of the new system (in rad/s)? 
(b) If a third such disk is dropped on the first two, find the final angular speed of the system (in rad/s).

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A disk of mass M is spinning freely at 6.99 rad/s when a second identical disk, initially not spinning, is dropped onto it so that their axes coincide. In a short time the two disks are corotating. HINT (a) What is the angular speed of the new system