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, what is the ratio of their moments of inertia I_Y/I_X equal to ?

Q: 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, what is the ratio of their moments of inertia IY/IX equal to ? (Both moments of inertia are about the axis passing through the centre of mass and perpendicular to plane of disc.)

(a) 64

(b) 81

(c) 49

(d) 36

Ans: (a)