Don't know how to solve this

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 500 pounds of phosphoric acid, 990 pounds of nitrogen, and 200 pounds of potash. If mix A costs $30 per cubic yard and mix B costs $40 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? If mix A costs $30 per cubic yard and mix B costs $40 per cubic yard, the farmer should blend yd3 of mix A and yd3 of mix B to meet the minimum monthly requirements at a minimum cost. The cost is $