Yearly minimum requirements for the solid sizes (in thousands) are 250, 150, 150, and 80, respec- tively. For the hollow sizes they are 190, 190, 160, and 150. The mills can operate up to three 40-hour shifts per week, 50 weeks a year. Present policy is that each mill must operate at least one shift. (a) Formulate a linear program to meet de- mand and shift requirements at minimum total cost using the decision variables