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Assume that there is a 100 liter tank.
It is initially filled with just distilled water.
It is continuously drained at R liters per minute.
This tank is always maintained full and homogeneous by continuously adding and stirring R liters per minute of an unknown acid (or some fancy oil if it helps you solving this problem).
How many liters W of water will be in the tank after M minutes?
Neglect any expansion or contraction when the acid is mixed with water.