11. A gardener requires fertiliser to have a minimum of 80 units of nitrogen, 15 units of potassium and 10 units of iron. Two fertiliser mixes are available. Mix X contains 20, 2 and 1 units of the minerals per kg, while Mix Y contains 4, 1, 2 units of the minerals per kg, respectively. Mix X costs 18 per kg while Mix Y costs 6 per kg. (a) Write down the inequality constraints and the equation of the cost function. (b) Graph the inequality constraints. Hence determine the number of kg of each mix which provides the minimum mineral requirements at minimum cost. (c) Confirm your answer in (b) algebraically.