Two plates of equal area are placed in contact with each other. Their thickness are 2.0 cmand 5.0 cm respectively. The temperature of the external surface of the first plate is -20 degree Celsius and that of the external surface of the second plate a 20 degree Celsius . What will the temperature of the contact surface with if the plate (1)are of the same material (2)have thermal conductivity in the ratio 2:5.