Two plates eachb of area A, thickness `L_1 and L_2` thermal conductivities `K_1 and K_2` respectively are joined to form a single plate of thickness `(L_1+L_2)`. If the temperatures of the free surfaces are `T_1 and T_2`. Calculate.

(a) rate of flow of heat
(b) temperature of interface and
(c) equivalent thermal conductivity.