What is the total r value for a wall made of 4 inches of wood, 2 inches of fiberglass, and 3 inches of foam? assume that the r values for a 1-inch thickness of these materials is given by: r(1" wood) = 2, r(1" fiberglass) = 4, and r(1" foam) = 5?

The total R value is computed from the R-values of the components in the stack as ...
  (inches of wood)*(R-value of wood) +(inches of fiberglass)*(R-value of fiberglass) +(inches of foam)*(R-value of foam)
  = (4 in)(2 /in) +(2 in)(4 /in) +(3 in)(5 /in)
  = 8 +8 +15
  = 31

The total R-value of the wall is 31.