Chapter 3 - 1D Steady-state Heat Conduction

The composite wall consists of three materials, two of which are of known thermal conductivity, k_A = 25 W/mK and k_C = 50 W/mK, and known thickness, L_A = 0.30 m and L_C = 0.20 m. The third material, B, which is sandwiched between materials A and C, is of known thickness 2 L_B, where L_B = 0.15 m, but unknown thermal conductivity k_B. This material is generating heat at a constant rate.
Under steady-state operating conditions, measurements reveal an outer surface temperature of T1 = 261C, and T2 = 211C. The ambient air temperature has a temperature of T = 25 C. The inside convection coefficient h is known to be 1000 W/m2 K. What is the
value of kB and the rate of heat generation?

[@nobelawasom7538]

how did you solve for Kb?