A brick wall of 20 cm thickness has thermal conductivity of 0.7 W m^-1K^-1. An insulation of thermal conductivity 0.2 W m^-1K^-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 insulation is