Consider a 25-m-long thick-walled concrete duct (k = 0.75 W/m · K) of square cross section. The outer dimensions of the duct are 20 cm × 20 cm, and the thickness of the duct wall is 2 cm. If the inner and outer surfaces of the duct are at 100°C and 30°C, respectively, determine the rate of heat transfer through the walls of the duct.

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30°C 100°C 16 cm -20 cm- || II 25 m 1 I 1 I I