The electrostatic force between two charges kept in air is F. If 30% of the space between the charges is filled with a medium, then the electrostatic force between the charges becomes $\frac{F}{2.56}$. The dielectric constant of the medium is
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When a medium of thickness $t$ and dielectric constant $K$ is introduced between two charges separated by distance $r$, the effective distance in the denominator of the force formula becomes $r_{eff} = (r-t) + t\sqrt{K}$. This is the key to solving such problems.