The outer temperature of the steel layer

A truck wall consists of a 1-mm-thick layer of steel (ks =15.1 W/mK) and a layer of polyurethane insulation (Kp = 0.025 W/mK). At steady state, a temperature decrease of 0.5°C occurs over the steel layer. The outer temperature of the steel layer is 100°C. From transportation perspective, the temperature of the inner surface of the polyurethane foam be no greater than -20°C. Determine (i) the rate of conduction, andii) the thickness of polyurethane foam that ensures this limit is met