Ice has formed on a shallow pond, and a steady state has been reached, with the air above the ice at -3.5°C and the bottom of the pond at 4.0°C. If the total depth of ice + water is 1.2 m, how thick is the ice? (Assume that the thermal conductivities of ice and water are 0.40 and 0.12 cal/m·C°·s, respectively.)