Problem Statement $2$:

$0\mathrm{.}5$ L of river water was collected just below the brewery. $20$ mL was diluted to $2$ L $,$ aerated and seeded. The dissolved oxygen content was $7\mathrm{.}0m\frac{g}{L}$ initially. After $5$ days, the dissolved oxygen content had dropped to $5\mathrm{.}0m\frac{g}{L}.$ A second sample was obtained $20$ days later and retested identically. The initial dissolved oxygen was $6\mathrm{.}9m\frac{g}{L}$ and, after $5$ days, dropped to $4\mathrm{.}5m\frac{g}{L}.$ Calculate UBOD for each sample. $($ k is $0\mathrm{.}23/$day$).$