Answer: Approximately 1.0714 gallons
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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.