The question is: explain the change in the market for skim milk, if demand for ice cream rises.
I'd say these two are unrelated in consumption (saying they're substitutes is kind of stretching it), but complements in production: you extract the cream from whole milk to make ice cream, and the rest becomes skim milk. (Right? Please correct me if I'm wrong.)
Based on this assumption, my answer is that the supply for skim milk will increase (the supply curve shifts right). As demand for ice cream rises, the demand curve for it shifts to the right, and assuming the supply curve for it remains still, this shift will increase both the price and quantity of ice cream. In turn, as ice cream prices rise, producers make more of it, and thus more skim milk.
The thing is, my textbook doesn't describe demand for a complement in production as a factor that shifts the supply curve. So I was wondering, is the reasoning above valid? Thanks.