The composite wall of an oven consists of three materials, two of which are of known thermal conductivity, kA = 25 W/m ⋅ K and kC = 60 W/m ⋅ K, and known thickness, LA = 0.40 m and LC = 0.20 m. The third material, B, which is sandwiched between materials A and C, is of known thickness, LB = 0.20 m, but unknown thermal conductivity kB. Under steady-state operating conditions, measurements reveal an outer surface temperature of Ts, o=20, an inner surface temperature of Ts, i=600°C, and an oven air temperature of T[infinity]=800°C. The inside convection coefficient h is known to be 25W/m^2⋅K. What is the value of kB?