A circular disc X of radius R is made from an iron plate of thickness t, and another disc Y of radius 4R is made from an iron plate of thickness t/4. Then the relation between the moment of inertia lX and IY is

Moment of Inertia of disc I=12MR2                                         =12πR2tρR2=12πtρR4 [As M=V×ρ=πR2tρ where t= thickness, ρ= density ∴ IyIx=tytxRyRx4              [ If ρ= constant ]⇒ IyIx=14(4)4=64            Given Ry=4Rx,ty=tx4⇒ Iy=64Ix