A box with a total surface area of 1.20 m 2 and a wall thickness of 4.00 cm is made of an insulating material. A 10.0 W electric heater inside the...

A box with a total surface area of 1.20 m2 and a wall thickness of 4.00 cm is made of an insulating material. A 10.0 W electric heater inside the box maintains the inside temperature at 20.0°C above the outside temperature. Find the thermal conductivity k of the insulating material.