Answer: 20 gallon or 11 gallon
Step-by-step explanation: let gallon of pure alcohol added to 15% mixture to make it up to 25% 15%alchol of 20 gallon can be shown as 15/100*20+× = (251 100) (×+20) 3+×=0.25× +5 0.75×=2 ×=2.66 gallon hence,2.66 gallon of pure alcohol need to added to 2 gallon 15% alcohol to obtain 25% solution.