The moment of inertia of a rigid body about an axis

We have two spheres,one of which is hollow and the other solid. They have identical masses and moments of inertia about their respective diameters. The ratio of their radii is given by:

Four identical uniform solid spheres,each of same mass '$M$' and radius '$R$',are placed touching each other as shown in the figure with centers $A, B, C, D$. If $I_{A}, I_{B}, I_{C}, I_{D}$ are the moments of inertia of these spheres respectively about an axis passing through their centers and perpendicular to the plane,then:

$A$ uniform solid cylinder of length $L$ and radius $R$ has a moment of inertia about its axis equal to $I_1$. $A$ small co-centric cylinder of length $L/2$ and radius $R/3$ is carved from this cylinder. The moment of inertia of this small carved cylinder about the same axis is $I_2$. The ratio $I_1/I_2$ is . . . . . . .

The ratio of the radii of gyration of a circular disc to that of a circular ring,each of same mass and radius,around their respective axes is

Four particles,each of mass $m$,are placed at the corners of a square of side $l$. The radius of gyration of the system about an axis passing through the center of the square and perpendicular to its plane is: