The wall of a cold storage consists of three layers- an outer layer of ordinary bricks, 25 cm thick, a middle layer of cork, 10 cm thick and an inner layer of cement, 6 cm thick. The thermal conductivities of the materials are 0.7, 0.043 and 0.72 W/m/K, respectively. The temperature of the outer surface of the wall is.30oC and that of inner is - 15oC