Answer:
Step-by-step explanation:
Each gallon of 7% milk contains 0.07 gallon of butterfat.
Each gallon of 4% milk contains 0.04 gallon of butterfat.
291 gallons of 6% milk contain 291×0.06 = 17.46 gallons of butterfat.
Let x be the number of gallons of 7% milk used. 291-x is the number of gallons of 4% milk used.
0.07x + 0.04(291-x) = 17.46
0.03x + 11.64 = 17.46
x = 200
Use 200 gallons of 7% and 91 gallons of 4%.