The walls of a refrigerator are typically constructed by sandwiching a layer of insulation between sheet metal panels. Consider a wall made from fiberglass insulation of thermal conductivity k; = 0.046 W/m ∙ K and thickness L_p = 50 mm and steel panels, each of thermal conductivity k_p = 60 W/m ∙ K and thickness L_p = 3 mm. If the wall separates refrigerated air at T_∞,i = 4°C from ambient air at T_∞,i = 25°C, what is the heat gain per unit surface area? Coefficients associated with natural convection at the inner and outer surfaces may be approximated as h_i = h_o = 5 W/m² ∙ K.