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 $$100^\circ C$$, 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 $$50^\circ C$$ on the outside of the plate surface. What is the thermal conductivity of the plate material?