Ahigh temperature muffle furnace uses a composite wall of three materials to manage heat transfer. inside the furnace, the air temperature is 800°c and the convection coefficient is 25 w/m2∙k. the inside surface temperature is 600°c. next, layer a has a thickness of 0.30 m and a conductivity of 20 w/m∙k. layer b has a thickness of 0.15 m. layer c has a thickness of 0.15 m and a conductivity of 50 w/m∙k. the outer surface temperature at steady state is 20°c. using an energy balance and a thermal circuit, determine the thermal conductivity of layer b.