hey guys, how to solve this word problem using gauss elimination method. u know using matrices and etc :P

the problem is :

A farmer can buy four types of plant food. Each barrel of mix A contains 30 pound of phosphoric acid, 50 pounds oof mitrogen and 20 pounds of potash; each barrel of mix B contains 30 pounds of phosphoric acid, 75 pounds of nitrogen . and 30 pounds of potash; each barrel of mix c contains 30 pounds of phosphoric acid, 25 pounds of nitrogen and 30 pounds of potash; each barrel of mix D contains 60 pounds of phosphoric acid, 25 pounds of nitrogen, and 50 pounds of potash. Sol tests indicate that a particular field needs 900 pounds of phosphoric acid. 700 pounds of nitrogen . and 820 pounds of potash. how many barrel of each type of food should the farmer mix together to supply the necessary nutrients for the field ?

Let X1 be the number of barrels of Mix A, X2 be the number of barrels of Mix B , and X3 be the number of barrels of Mix C, and X4 be the number of barrels of Mix D.

i just need help with the equations to start with.

and please tell me how do u decide those equations also ? ty alot for everybody`s time.