ABC Electricity owns a coal gasifier plant. The coal gasifiers on the plant can use three types of coal (A, B and C) to produce two qualities of producer gas (X and Y). There are two processes; a new process and an old process, which are available to use blended coal. For each production run, the old process uses 20 units of coal A, 28 units of coal B and 8 units of coal C to produce 24 units of X and 20 units of Y. For the new process, 12 units of coal A, 36 units of coal B and 16 units of coal C are used to produce 20 units of X and 28 units of Y. The gasifier plant is required to produce at least 1800 units of X and at least 1600 units of Y. However, it has only 2000 units of coal A, 2500 units of coal B and 1500 units of coal C. For each unit of X, the company earns $4000 and for Y it receives $6000 per unit. Formulate the linear programming model which determines the required number of production runs to maximise ABC Electricity's revenue for the required production