4.A contractor may purchase material from two different sand and gravel pits. The unit cost of material including delivery from pits 1 and 2 is $ 50 and $ 70 per cubic meter, respectively, the contractor requires at least 100m3 of mix. The mix must contain a minimum of 30% sand. Pit 1 contains 25% and pit 2 contains 50% sand. If the objective is to minimize the cost of material, define the decision variables and formulate a mathematical model. a) Draw the feasible region b) Determine the optimum solution by the graphical method c) Use TORA to model and solve this problem