The formula for the electrostatic force between two charges is given by:

Where F is the force (in N), K is the electrostatic constant, q1 and q2 are the charges (in C) and d is the distance between them (in m).
If the force is 100 N for a distance of 1 m, we have:

Now, doubling the distance, we have:

Therefore the new electrostatic force is equal to 25 N.