Answer:

separation between two charges is 2r

Explanation:

given:

let the two charges be q1 and q2

two charges separated by the distance r1

force between two charges is f

we know that

f=k(q1.q2)/r1^2

if the force is f/4 then f2,

f/4=k(q1.q2)/r2^2

f2/f1

(f/4)/f=[k(q1.q2)/r2^2]/[k(q1.q2)/r1^2]

1/4=r1^2/r2^2

r2^2=4r1^2

r2=2r1