Repeat the refinery model in Exercise 4 with new demand levels of 500 gallons heating oil, 300 gallons diesel oil, and 600 gallons gasoline. Try to find an approximate solution (within 30 gallons) with this set of demands. Something is going wrong and there is no valid set of pro-Sec. 1.2 Systems of Linear Eq nations 19 duction levels to attain this set of demands. What is invalid about the solution to this refinery problem?