1. The problem statement, all variables and given/known data

Hi

I need clarification on this problem. Calculate

a) the force between 2 charges of +1.4nC and +1.6nC on point conductors 40cm apart in air.

b)What size of charge on a third point conductor placed midway between the first two conductors would result in doubling of the magnitude of the force on the 1.4nC charge?

2. Relevant equations

Coulombs law= (Kq1q2)/r^2

3. The attempt at a solution

I obtained 1.25 x 10^7N for the first part. For the second part, I stated the new charge as q and the new distance as 20cm. Making the new charge the subject of my formula, i put in the new details. But my result is not correct.

Please any advise. Thanks