A composite plane wall consists of a 5-in.-thick layer of insulation (ks = 0.029 Btu/h*ft*°R) and a 0.75-in.-thick layer of siding (ks = 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/h*ft^2 of surface area.