An unheated garage is placed on the north wall of a building to act as a thermal buffer zone. If the garage has roof area Ar, window area Awi, door area Ad, and wall area Awa, what is the effective U value for the north wall of the building if its area is An? The garage floor is well insulated and has negligible heat loss. Express the effective U value in terms of the U values and areas of the several garage surfaces.