The figure shows the cross-section of the outer wall of a house built in a hill - resort to keep the house insulated from the freezing temperature of outside. The wall consists of teak wood of thickness L1 and brick of thickness (L=5L1). sandwiching two layers of an unknown material with identical thermal conductivities and thickness. The thermal conductivity of teak wood is K1 and that of brick is (K2=5K). Heat conduction through the wall has reached a steady-state with the temperature of three surfaces being known. (T1=25∘C,T2=20∘C and T5=−20∘C). Find the interface temperature T4 and T3.