The composite wall of an oven consists of three materials, two of which are of known thermal conductivity, kA =20 W/m. K and kC =50 W/m. K, and known thickness, LA =0.30 m and LC =0.15 m. The third material, B, which is sandwiched between materials A and C, is of known thickness, LB= 0.15 m, but unknown thermal conductivity kB. Under steady-state operating conditions, measurements reveal an outer surface temperature of Ts, o =20C, an inner surface temperature of Ts, i=600C, and an oven air temperature of T=800C. The inside convection coefficient h is known to be 25 W/m2.K. What is the value of kB?