A: The wall and window are in parallel heat flow paths, not series, so you can't simply add resistances — parallel resistances add as inverses. Working with U-values (conductivities) alongside their respective areas makes the weighted average formula more intuitive, because it works like a mixing calculation: (U_wall × A_wall + U_window × A_window) / A_total. You could do it with R-values too, but the algebra gets less clean — either way, the answer must reconcile to the same total resistance.

A: The problem stated a temperature differential across the wall surfaces, meaning those temperatures are measured at the physical inner and outer surfaces — so no film layer is needed in the thermal network. When a problem gives you ambient indoor and outdoor air temperatures instead, that's when you'd add film resistance, and in that case the problem should provide the convection coefficients. If the film coefficients aren't given, it's a strong signal they're not expected — but it's not a guarantee, so read carefully for whether temperatures are at surfaces or in the air.