A composite plane wall consists of a 3-in.-thick layer of insulation (κs = 0.029 Btu/h · ft · °R) and a 0.75-in.-thick layer of siding (κs = 0.058 Btu/h · ft · °R). The inner temperature of the insulation is 67°F. The outer temperature of the siding is −8°F. Determine at steady state (a) the temperature at the interface of the two layers, in °F, and (b) the rate of heat transfer through the wall in Btu per ft2 of surface area.