Hey buddy, In this question you may proceed by equating angular momentum of both side. Angular momentum in the first case is equal to angular momentum of second case.
1/2Iw^2=1/2(I1+I2)w1^2
mr^2w^2=(mr^2+m(r/2)^2)w1^2
r^2w^2=5/4r^2w1^2
4/5w^2=w1^2
4/5*100=w1^2w1=root80
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