Coulomb's law states that F = k(q1)(q2)/r^2, where k = 9 x 10^9 N-m^2/C^2
From the given:
81 = (9x10^9)(2x10^-5)(4.5x10^-4)/r^2
r^2 = 1
r = 1 m
Since both charges have the same sign (positive), the force is repulsive (like charges repel).
Therefore there is a 1-m distance between the two charges, and the force is repulsive.
