For the next seven questions of the exam, consider Babs and Donna, who can each produce cake, beer, or some combination of the two using only 40 hours of labor each.Babs can produce a maximum of 30 cakes if she produces no beer, and 80 units of beer if she produces no cake. Donna can produce a maximum of 20 cakes with no beer produced; if she makes no cake, she can produce 40 units of beer.

Calculate Donna's opportunity cost for cake.