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 rotation between the moment of inertia I_x and I_y is : 

I_y = 64 I_x

Disc x : radius = R ; thickness = t

Disc y : radius = 4R ; thickness = (t/4)

Moment of Inertia = [(MR²) / 2]

∴ (I₁ / I₂) = (M₁ / M₂) (R₁ / R₂)² ... (1)

also mass = density × volume

M = ρ ∙ V

M = ρ ∙ πR²t

material is same hence density is same.

(M₁ / M₂) = (R₁ / R₂)² ∙ (t₁ / t₂)

  = [R / (4R)]² ∙ [t / (t/4)]

  = (1 / 16) × 4 = (1/4)

From (1) : (I₁ / I₂) = (1/4) × [R / (4R)]²­ 

(I₁ / I₂) = (1/4) × (1 / 16) = (1 / 64)

i.e I_y = 64 I_x