Geometric series/Contaminant/Exercise

Into a settling pond with a capacity of ${}2000\,\mathrm {m} ^{3}$, at the beginning of each day, ${}200\,\mathrm {m} ^{3}$ water is filled in. It contains ${}10\,\%$ of a certain contaminant, and is mixed completely with the water. During each day, by a biological process, the given amount of contaminant is reduced by ${}20\,\%$. At the end of each day, ${}200\,\mathrm {m} ^{3}$ water leaves the pond. Which percentage of the contaminant

will there be in the drain, in the long run, if the pond is filled at the beginning with ${}1800\,\mathrm {m} ^{3}$ clear water?

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