A producer of fertiliser receives an order to supply exactly 5000 kg of fertilizer, consisting of a special mixture of phosphate and ammonium nitrate. The mixture must not contain more than 1500 kg of phosphate and not less than 750 kg of ammonium nitrate. The cost per kilogram of phosphate and ammonium nitrate are R6 and RS respectively. The producer would like to determine the ratio in which raw materials must be combined so as to limit costs to minimum. Formulate a linear programming model for the above problem and hence find the optimal solution by using the simplex method"