I'm confused by the solution, it gives us the total resistance, but the overall heat coefficient. But the solution has us use 1/R. Even the units are for R, not U. So I took the percents x the R- values, so .18*9.7+.82*21.5. Why do I need to solve for this in the U form? A wood-stud wall has batt insulation between the wall studs. The total resistance value for the insulated portion of the wall is 21.5 hr-ft^2-F/Btu, and the value for the studded portion is 9.7 hr-ft^2-F/Btu. The studs, plates, and headers occupy 18% of the wall area. Assuming only parallel heat flow, the average total resistance (hr-ft^2-F/Btu) for the wall is most nearly: Answers A. 19.4 B. 17.6 C. 15.6 D. 13.4