A health food store packages a nut sampler consisting of walnuts, pecans, and almonds. Suppose that each ounce of walnuts contains 12 units of protein and 3 units of iron and costs 12 cents, that each ounce of pecans contains 1 unit of protein and 3 units of iron and costs 9 cents, and that each ounce of almonds contains 2 units of protein and 1 unit of iron and costs 6 cents. If each package of the nut sampler is to contain at least 24 units of protein and at least 18 units of iron, how many ounces of each type of nut should be used to minimize the cost of the sampler