No, the prices are NOT the constraints. The constraints are "The farmer needs a minimum of: 460gr of fats, 960 grams of carbs and 220 grams of proteins for his horses."
You are told that
"Each bag of Lingo contains: 20gr of fats, 30gr of carbs and 5gr of proteins.
Each bag of Bingo contains: 10gr of fats, 30gr of carbs and 10gr of proteins."
So if you buy X bags of Lingo and Y bags of Bingo you are supplying 20X+ 10Y grams of fats, 30X+ 30Y grams of carbs, and 5X+ 10Y grams of proteins. Since, again, "The farmer needs a minimum of: 460gr of fats, 960 grams of carbs and 220 grams of proteins for his horses."
he must have , , and .
The question "How many of each bags should the farmer purchase to satisfy the minimum requirements?" doesn't ask anything about cost of the bags.
I would solve this by graphing 20X+ 10Y= 460, 30X+ 30Y= 960, and 5X+ 10Y= 220, calculating where they intersect and finding the lowest vertex of the region above all the lines.