A Concrete Wall of a Cold Storage Room Measures $- 10 ^ { \circ } \mathrm { C }$

A concrete wall of a cold storage room measures 3.0 m high, 5.0 m wide, and 20 cm thick. The inside wall is to be covered by a layer of wood in order to reduce the rate of heat flow through the wall by 90 percent. The inner surface of the wooden wall is maintained at $- 10 ^ { \circ } \mathrm { C }$ and the outer surface of the concrete wall is at $20 ^ { \circ } \mathrm { C }$ The thermal conductivities of concrete and wood are 0.80 $\mathrm { W } / \mathrm { m } \cdot \mathrm { K }$ (concrete) and 0.040 $\mathrm { W } / \mathrm { m } \cdot \mathrm { K }$ (wood) . What should be the thickness of the layer of wood?

A) 60 mm
B) 100 mm
C) 90 mm
D) 70 mm
E) 80 mm

Correct Answer:

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