An animal feed to be mixed from soybean meal and oats must contain at least 168lb of protein, 24lb of fat, and 14lb of mineral ash. Each sack of soybeans cost $ 21 and contains 70lb of protein, 8lb of fat, and 7lb of mineral ash. Each sack of oats costs $7 and contains 21lb of protein, 7lb of fat, and 1 lb of mineral ash. How many sacks of each should be used to satisfy the minimum requirements at minimum cost?

What is the minimum cost?