Should've put this in mathematics, you know? But don't worry, mistakes happen!
Though, in order to do this problem, we need to add 20 for the total gallon.
Let's use X for a placeholder to determine the amount, as the gallon of 25% alcohol and percentages can be in decimal forms. It'd be 0.25x gallons worth of alcohol afterwards. We then add 20-x gallons to 5% of alcohol, which is 0.05(20-x).
This would then allow us to find the sum of 20 gallons to 20% of alcohol, containing at least 0.2(20) gallons of the alcohol.
We'd then be able to develop this equation;
0.25x + 0.05(20-x) = 0.2(20).
And then so on the sequence continues...
0.25x + 1 - 0.05x = 4
0.2x = 3
x = 3/0.2
We'd then be able to come to a conclusion that;
1. X equals 15 gallons of 25% alcohol
2. 20-x equals 5 gallons of 5% alcohol.