Someone is walking outside when the temperature of the air is -10oC. The metabolic rate of walking is 140 Cal/m2hr, and the usual muscle efficiency (20%) applies. They are completely covered by clothing with a coefficient of thermal conductivity of 0.36 Cal-cm/m2 hr oC. Their total surface area is 1.7 m2, and this area can be used as the area through which heat is being conducted away from the body. Their skin temperature is the same as the inside surface of the clothing, 35oC. The temperature of the outside of the clothing is -10oC. It is dark outside. Their mass is 75 kg. Ignore heat lost to breathing. How thick does the down have to be to maintain a constant internal body temperature of 37oC?