I think this is all coulomb's law. So:
|F| = k \frac{q_1 q_2}{r^2}
a) Increase separation, decrease force on each by inverse square law.
So for distance r, we have F.
Distance 2r, distance is doubled, so we get 1/2^2 = 1/4, so the force is F/4
b) Same thing, but we get 9F.
c) Charge is...