A farmer can buy two types of plant​ food, mix a and mix
b. each cubic yard of mix a contains 20 pounds of phosphoric​ acid, 30 pounds of​ nitrogen, and 5 pounds of potash. each cubic yard of mix b contains 10 pounds of phosphoric​ acid, 30 pounds of​ nitrogen, and 10 pounds of potash. the minimum monthly requirements are 440 pounds of phosphoric​ acid, 990 pounds of​ nitrogen, and 200 pounds of potash. if mix a costs ​$20 per cubic yard and mix b costs ​$30 per cubic​ yard, how many cubic yards of each mix should the farmer blend to meet the minimum monthly requirements at a minimum​ cost? what is this​ cost?