In a production process model such as Repco’s, certain inputs make no sense in the usage table (the range B7:D9 of the model). For example, suppose that, in addition to current usages, each unit of drug A requires one unit of drug C. Why does this result in a nonsensical problem? What happens if you run Solver on it anyway? What happens if you run Solver on it after adding a constraint that the sum of the units produced (over all three drugs) must be at least 1?