(a)
Charge on sphere A,
Charge on sphere B,
Distance between the spheres,
Force of repulsion between the two spheres,
Where, Free space permittivity
Therefore, the force between the two spheres is
(b)
After doubling the charge,
Charge on sphere A,
Charge on sphere B,
Now, if the distance between the sphere is halved, then
Force of repulsion between the two sphere,
Therefore, the force between the two sphere is approximately 0.243 N.