For a circular disc made of material with density ρ, moment of inertia is I=21MR2=21ρπR2T⋅R2=21ρπR4T.
For disc 1: I1=21ρπR14T1
For disc 2: I2=21ρπR24T2
Given I1=I2:
R14T1=R24T2
With R2R1=2:
(2R2)4T1=R24T2
16R24T1=R24T2
T2T1=161
Therefore α=16.