If there is an electric force of 5 newtons between two charged particles (q1 = + 10 C and q2 = + 10 C), how far apart are the particles?

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