How should the distance, D, between two point charges, q1 and q2, be changed to double their electric potential energy

The electrostatic potential energy, U, of one point charge q at position d in the presence of an electric field E is defined as the negative of the work W done by the electrostatic force to bring it from the reference position d to that position

Thus, to double the electric potential energy U we need to reduce the distance of separation by half (1/2) because they are inversely proportion