The composite wall of an oven consists of three materials A, B, and C. Material A has a thermal conductivity value of 20 ∙℃ and a thickness of 0.3m. Material C has a thermal conductivity value of 50 ∙℃ and a known thickness of 0.15m. The third material, B, is sandwiched between materials A and C. Material B has a known thickness of 0.15m, but an unknown thermal conductivity. The outer surface temperature of the oven is 20°C and the inner surface temperature is 600°C. The air inside the oven is 800°C with a convection coefficient of 25 2∙℃ . Assuming no convection on the outside of the oven, what is the value of thermal conductivity for material B?