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.
