A concrete wall of a cold storage room measures 3.0 m high, 5.0 m wide, and 20 cm thick. The room temperature is maintained at -10°C and the outside temperature is 20°C The inside wall is to be covered by a layer of wood in order to reduce the rate of heat loss through the wall by 90 percent. The thermal conductivities of concrete and wood are 0.80 W/m • K and 0.040 W/m • K, respectively. Under steady state conditions, the thickness of the layer of wood required is closest to

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

The answer is 90 mm but I have no idea how to get there. I've tried this problem four times but no luck.