A brick wall of $20\:cm$ thickness has thermal conductivity of $0.7\:W\:m^{-1}\:K^{-1}$. An insulation of thermal conductivity $0.2\:W\:m^{-1}\:K^{-1}$ is to be applied on one side of the wall, so that the heat transfer through the wall is reduced by $75\%$. The same temperature difference is maintained across the wall before and after applying the insulation. The required thickness (in $cm$) of the isulation is ________________