In a mixture of 80~\text{litres} of a liquid and water, 25\% of the mixture is the liquid. How much water should be added to the mixture so that the liquid becomes 20\% of the mixture?

Explanation

The initial mixture contains 25\% of 80 = 20~\text{litres} of liquid. If x~\text{litres} of water are added, the total volume becomes 80+x. The liquid amount remains 20~\text{litres}. Setting \frac{20}{80+x} = 20\% = \frac{1}{5} gives 100 = 80 + x \implies x = 20.

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