Compute a mixed$-$hybrid production plan using a combination of inventory, overtime, subcontracting, hiring and firing and calculate its total cost. For this plan, the maximum number of employees that can be hired or laid$-$off in a single period at one time is $50($this is only per period; not the entire plan$).$ Furthermore, subcontractors can only supply up to $40$ additional units per month and each employee can produce $5$ extra units per month on overtime. Also, they are not allowed to use overtime nor subcontracting to build inventory. This mixed$-$strategy hybrid plans cost MUST be less than the previous two plans.