The wall of a furnace is made up of inside layer of silica brick 120 mm thick covered with a layer of magnetite brick 240 mm thick. the temperature at the inside of the silica brick wall and the outside surface of magnetite brick wall are 7250c and 1100c respectively. the contact thermal resistance between the two walls at the interface is 0.00350c/w per unit wall area. if the thermal conductivities of the silica and magnetite bricks are 1.7 w/m0c and 5.8 w/m0c, calculate: a. the rate of heat loss per unit area of walls, and b. the temperature drop at the surface.