a 6-in. insulated frame wall of a house has an average thermal conductivity of 0.04 btu/h ⋅ ft ⋅ °r. the inner surface of the wall is at 68°f, and the outer surface is at 40°f. the temperature varies linearly through the wall. determine at steady state the rate of heat transfer through the wall, in btu/h. if the wall is 20 ft × 10 ft, determine the total amount of energy transfer in 10 hours, in btu.