You are doing an experiment to find out $\mathrm{BOD}_5$ of a wastewater. You have taken $25 \mathrm{~mL}$ wastewater having ultimate BOD of $75 \mathrm{mg} / \mathrm{L}$ and placed it in $300 \mathrm{~mL}$ BOD bottle and filled it with dilution water. The initial DO of the diluted sample is $6.5 \mathrm{mg} / \mathrm{L}$. On the $5^{\text {th }}$ day, you were not able to measure the DO due to some unavoidable circumstances. However, the DO at the end of the $7^{\text {th }}$ day is found to be $1.25 \mathrm{mg} / \mathrm{L}$. Assume all the experiments are done at the same temperature, and no biodegradable organics are present in the dilution water. The $\mathrm{BOD}_5$ of the wastewater sample is __________ (in $\mathrm{mg} / \mathrm{L}$, $\text{rounded off to two decimal places).}$
