An animal feed to be mixed from soybean meal and oats must contain at least 168 lb of protein, 27 lb of fat, and 14 lb of mineral ash. Each sack of soybeans costs $ 21 and contains 70 lb of protein, 9 lb of fat, and 7 lb of mineral ash. Each sack of oats costs $7 and contains 21 lb of protein, 7 lb of fat, and 1 lb of mineral ash. How many sacks of each should be used to satisfy the minimum requirements at minimum cost?