Let the mass per unit area be σ. Then the mass of the complete disc =σ[π(2R)2]=4πσR2 The mass of the removed disc =σ(πR2)=πσR2 So mass of the remaining disc =4πσR2−πσR2=3πσR2 Let center of mass of 3πσR2 mass is at x distance from origin 0 ∴
3πR2σ.x+πR2σ.R
4πR2σ
=0 As center of mass of full disc is at Origin. ∴x=−