The surest concrete company produces concrete in a continuous process. two ingredients in the concrete are sand, which Surest purchases for $6 per ton, and gravel, which costs $8 per ton. Sand and gravel together must make up exactly 75% of the weight of the concrete. Furthermore, no more than 40% of the concrete can be sand and at least 30% of the concrete must be gravel. Each day 2000 tons of concrete are produced. Formulate as a linear programming problem.