^ 131 Iis an isotope of Iodine that β decays to an isotope of Xenon...

${}^{131}Iis$ an isotope of Iodine that $\beta$ decays to an isotope of Xenon with a half-life of $8days$ . A small amount of a serum labelled with ${}^{131}I$ is injected into the blood of a person. The activity of the amount of ${}^{131}I$ injected was $2.4 \times {10}^{5}$ Becquerel (Bq). It is known that the injected serum will get distributed uniformly in the blood stream in less than half an hour. After 11.5 hours, 2.5 ml of blood is drawn from the person's body, and gives an activity of $115Bq$ . The total volume of blood in the person's body, in litres is approximately (you may use $\mathrm{e}^{\mathrm{x}} \approx 1+\mathrm{x}$ for $|\mathrm{x}|<<1$ and $\ln 2 \approx 0.7$).