Force between two point charges q _ 1 and q _ 2 placed in vacuum at...

Using Coulomb's law, we can write:

In air $F = \dfrac{1}{4\pi{ε}_{0}}\dfrac{{q}_{1}{q}_{2}}{{r}^{2}}$

In medium $F ' = \dfrac{1}{4\pi\left(K{ε}_{0}\right)}\dfrac{{q}_{1}{q}_{2}}{{\left(r '\right)}^{2}} = \dfrac{1}{4\pi\left(5{ε}_{0}\right)}\dfrac{25{q}_{1}{q}_{2}}{{\left(r\right)}^{2}} = 5F$