As indicated by the algebraic model of the Pigskin problem, there is no real need to calculate inventory on hand after production and constrain it to be greater than or equal to demand. An alternative is to calculate ending inventory directly and constrain it to be nonnegative.

Modify the current spreadsheet model to do this. (Delete rows 16 and 17, and calculate ending inventory appropriately. Then add an explicit nonnegativity constraint on ending inventory.) Is the optimal solution to this modified model the same as before? It should be.