The Dervish Chemical Company operates a small plant for the manufacture of two joint chemical products X and Y. The production of these chemicals requires two raw materials, A and B, which cost R5 and R8 per litre respectively. The maximum available supply per week is 3600 litres of A and 2000 litres of B. The plant can operate using either of two processes, which have differing operating costs and raw materials requirements for the production of X and Y, as follows. The plant can run for 120 hours per week in total but, for safety reasons, process 2 cannot be operated for more than 80 hours per week. The selling prices of output are: X R18 per litre Y R24 per litre Required: Formulate a linear programming model, and then solve it graphically to determine how the plant should be operated each week