How many gallons of pure alcohol should be mixed with 20 gal of a 15?

Let x gallon of pure alcohol added to 15 % mixture to make it upto 25 %

15 % alcohol of 20 gallon can be shown as

(15 / 100) * 20 + x = (25 / 100) (x + 20)

3 + x = 0.25 x + 5

0.75 x = 2

X = 2.66 gallon

Hence, 2.66 gallon of pure alcohol need to added to 20 gallon 15 % alcohol to obtain 25 % solution.