Question

Two charges each with a magnitude of 1.1⋅10−7 C experience a force of 4.2⋅10−4 N. How far apart are the charges from each other? 

Answer 1

$F=kq^2/d^2$

$d=q\sqrt{\frac{k}{F}}=1.1\cdot10^{-7}\sqrt{\frac{9\cdot10^9}{4.2\cdot10^{-4}}}=0.5\ m$