Lee would need to add 5 liters to have a mixture with 50% water and 50% solvent.
It is known the initial mixture contains 25 liters and that it has 60% water and 40% solvent.
Water liters:
Solvent liters
Now, a 50% rate for both substances implies there is the same number of liters. This means to reach this balance Lee needs to add 5 solvent liters. This would make a mixture with 15 water liters and 15 solvent liters.
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