Best answer

Let x be the amount in litres of the 100% alcohol (pure alcohol).

10% of 7 litres is 0.7L alcohol. There are x litres of pure alcohol.

The total quantity of the mixture is 7+x litres.

The amount of alcohol is 0.7+x=0.3(7+x) because the mixture contains 30% alcohol.

So 0.7+x=2.1+0.3x, 0.7x=2.1-0.7=1.4, x=1.4/0.7=14/7=2L.

2 litres of pure alcohol are needed to make a 30% alcohol mixture.

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