An electric heater is sandwiched between two plates each 0.3 m long and 0.1 m wide with a thickness of 30 mm. At steady-state condition, the heater is maintained at a temperature of 100°C, with a current of 0.25 A and voltage of 200 V. Assume the plates are perfectly insulated at the edges, and the heater is having perfect contact with the plates to give a temperature of 50°C on the outside of the plate surface. What is the thermal conductivity of the plate material?