The Solution

The Solution
Or, 12
(4) = a -[(a+b)/12] Σ[ θ(Wi – x) + (Wi – x)δ(Wi – x)] i=1
 
12
= a – [(a+b)/12] n - [(a+b)/12] Σ (Wi – x)δ(Wi – x)
i=1
 
= a – [(a+b)/12] n
where n = number of months for which Wi >x