Answer:
8 gallons at $7
12 gallons at $2
Step-by-step explanation:
x gallons at $7 with the remaining (20 - x) gallons at $2 to get 20 gallons at $4
x(7) + (20 - x)(2) = 20(4)
Distribute
7x + 40 - 2x = 80
Combine like terms
5x + 40 = 80
Subtract 40 from both sides
5x = 40
Divide both sides by 5
x = 8 gallons at $7
20 - x = 12 gallons at $2