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in a BOD test using $5\%$ dilution of the sample $(15ML$ of sample and $285mL$ of dilution water$),$ dissolved oxygen values for the sample and dilution water blank bottles after five days incubation at $20C$ were $3\mathrm{.}80$ and $8\mathrm{.}80m\frac{g}{L}.$ respectively. Dissolved oxygen originally present in the undiluted sample was $0\mathrm{.}80m\frac{g}{L}.$ The $5-$ day $20C$ BOD of the sample is