Answer:
0.66 times the original one
Explanation:
A disc of moment of inertia 2I is rotating with angular speed ω. Now another disc of moment of inertia I dropped on it and the new angular velocity is ω'.
By use of conservation of angular momentum
2I x ω = (2I +I ) x ω'
ω' = 2ω/3
ω' = 0.66 ω
Thus, the new angular velocity becomes 0.66 times the original one.