How many liters each of a 15% acid solution and a 25% acid solution must be used to produce 80 liters of a 20% acid solution
Let x represent the number of liters of the 15% acid solution.
Then 80 - x represents the number of liters of the 25% acid solution.
The total amount of acid in the 15% solution is 0.15x liters.
The total amount of acid in the 25% solution is 0.25(80 - x) liters.
The total amount of acid in the 20% solution is 0.20(80) = 16 liters.
0.15x + 0.25(80 - x) = 16
0.15x + 20 - 0.25x = 16
-0.10x = -4
x = 40
Therefore, 40 liters of the 15% acid solution must be used and 80 - 40 = <<80-40=40>>40 liters of the 25% acid solution must be used. Answer: \boxed{40}.