f = k q1 q2 / d²
a) (9 10⁹ N·m²/C² (30 10⁻⁶ C)²) / (0.5 m)²
b) (9 10⁹ N·m²/C² (30 10⁻⁶ C)) / 0.5 m
c) (9 10⁹ N·m²/C² 30 10⁻⁶ C) / 0.5 m
d) (9 10⁹ N·m²/C² 30 10⁻⁶ C²) / (0.5 m)²