Let P be the product solution, A be the 30% alcohol solution and W be the distilled water.
If we multiply the percentage of alcohol to the volume of the stream, we can obtain the amount of alcohol. Alcohol in the product is the sum of the alcohol in the solution and water.
Vp x 0.25 = 50 x 0.3 + Vw x 0
Vp = 60 gallons of product solution
Volume of product = volume of alcohol sol + vol of water
60 = 50 + Vw
Vw = 10 gallons
10 gallons of water must be added.