Are you familiar with Coulomb's law? For two point charges with charges
q1,q2 of opposite sign positioned a distance
r apart, there is an attractive force between the two that satisfies:
Force=4πϵ0r2∣q1q2∣Apply this to the case
Force=0.5F,r=20 to find
F in terms of
∣q1q2∣. Then apply it to the case
Force=F,r=d, substitute what you found in the previous step, and rearrange to find
d.
Edit: Oh, almost forgot the
.