Two plates A and B have thermal conductivities 84W m ^ - 1 K ^ - 1...

Two plates A and B have thermal conductivities $84W{m}^{- 1}{K}^{- 1}$ and $126W{m}^{- 1}{K}^{- 1}$ respectively. They have same surface area and same thickness. They are placed in contact along their surfaces. If the temperatures of the outer surfaces of A and B are kept at ${100}^{∘}C$ and ${0}^{∘}C$ respectively, then the temperature of the surface of contact in steady state is_____ $C ∘$.