Of the charge Q on a tiny sphere, a fraction d is to be transferred to a

Question:

Of the charge Q on a tiny sphere, a fraction d is to be transferred to a second, nearby sphere. The spheres can be treated as particles.
(a) What value of a maximizes the magnitude F of the electrostatic force between the two spheres?
What are the
(b) Smaller and
(c) Larger values of a that put F at half the maximum magnitude?