The Brittle Bird Store prepares birdseed mixtures by mixing sunflower seeds and millet and packaging them as 5-pound bags for sale. If sunflower seed (s) is regularly sold for $1.75 per pound, and millet (m) is sold for $2.50 per pound, what system of equations could be used to calculate how many pounds of sunflower seed is added to the mixture if the store sells the mixture for $15?