if you have 3 gallons of 95% alcohol, how much pure water should be added to be able to produce 70% alcohol?

 Approximately 1.0714 gallons

Work Shown:

x = amount of pure water

3 gallons of 95% alcohol means you have 3*0.95 = 2.85 gallons of pure alcohol.

If we add on x gallons of pure water, we'll have 3+x gallons of solution

So we'll have 2.85/(3+x) representing the ratio of the amount of pure alcohol to total solution. Set this equal to 0.70 and solve for x

2.85/(3+x) = 0.70

2.85 = 0.70(3+x)

2.85 = 0.70(3)+0.70(x)

2.85 = 2.10+0.70x

2.85-2.10 = 0.70x

0.75 = 0.70x

0.70x = 0.75

x = 0.75/0.70

x = 1.07142857142858 which is approximate

x = 1.0714

So if you add about 1.0714 gallons of pure water, you'll go from 95% alcohol to 70% alcohol.