Using the Coulumb law,
Force (electric) = [k * Q1 * Q2] / d², "d" is "how far apart are the particles"...
F = 5 N
Q1 = 10 C
Q2 = 10 C
5 = k*(10)² / d²
5*d² = 100*k
d² = 20*k
k is the Coulomb's constant : 9*10^9 Nm²/C²
d² = 20*(9*10^9)
d² = 18*10^10
d = 4.2*10^5 m or 420 km
