A barrel contains 25 liters of a solvent mixture that is 40% solvent and 60% water. Lee will add pure solvent to the barrel, without removing any of the mixture currently in the barrel, so that the new mixture will contain 50% solvent and 50% water. How many liters of pure solvent should Lee add to create this new mixture?

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We can calculate the amount of solvent and water of the mixture:

$$ \text{Solvent} = 25(0.4) = 10 $$ $$ \text{Water} = 25-10 = 15 $$

We will need to add \(\boxed{5}\) liters of solvent so that the mixture contains the same amount of each.