## Conceptual Physics (12th Edition)

$q = 2.8 \times 10^{-6} C$
Use Coulomb's Law, $F = k \frac{q_{1} \; q_{2}}{d^{2}}$. $$20 N = 9.0 \times 10^{9} \frac{N m^{2}}{C^{2}} \frac{q^{2}}{(0.06 m)^{2}}$$ $$q^{2} = 8.0 \times 10^{-12} C^{2}$$ $$q = 2.8 \times 10^{-6} C$$