Finding center of mass of sphere with a spherical cavity removed.

Hi there,

  I would be very grateful to anybody who can solve the following question for me (and show their working as to how they solved the problem):

 A spherical cavity of radius r is made inside a uniform solid sphere of radius R, in such a way that the two spherical surfaces touch at the point P. If G is the centre of mass of the remaining material,

 find(i) the distance PG,

and (ii) the radius of gyration of the remaining material about a common tangent at P to the spherical surfaces.