A closed box is filled with dry ice at a temperature of -76.5 °C, while the outside temperature is 21 °C. The box is cubical, measuring 0.36 m on aside, and the thickness of the walls is 0.085 m. In 1 hour 600,000 J of heat is conducted through the six walls. Find the thermal conductivity of the material of the box.

$\displaystyle {k}$ = J/(s*g*°C)