Two thin discs, each of mass M and radius r meter, are attached as shown in the figure, to form a rigid body. The rotational inertia of this body about an axis perpendicularto the plane of disc B passing through its center is

Moment of inertia of discs A and B about the axis through their center of mass and perpendicular to the plane will be IAA=IBB=12Mr2Now, moment of inertia of disc A about an axis through B, by theorem of parallel axis will be IAB=IAA+M(2r)2=12Mr2+4Mr2=92Mr2 So I=IBB+IAB=12Mr2+92Mr2=5Mr2