Volume of a sphere is 4/3Πr³. If a hole of radius r is bored through, the hole with generate a circular shape in the sphere. The volume of the remaining portion of the sphere will be the difference between the volume of the sphere and the area of the hole bored(which will be volume of a cylinder since the hole bored will create a cylindrical shape in the sphere)
Area of the remaining portion = Volume of sphere - volume of a cylinder
For Total energy and momentum to be conserved, the minimum energy of the photons released is equal to twice the rest mass energy of an electron that is
The annihilation of electron -positron cannot produce a single photon. It is prohibited by the law of conservation of energy and momentum.