A 20-litre reservoir is filled wit water.5 litres are removed and replaced with pure grape drink.5 litres of ?!
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Sorry I only work in Gallons.
Anyway from the way the problem has been worded The first mix of grape juice and water are replaced with pure grape juice so no matter how many times you change it it will still be grape juice 100% but as grape juice contains water naturally then the answer is approximate.