Given data
*The half-life of iodine-131 is t = 8.07 days
*The amount of quantity left is N = 0.25 g
*The number of days is T = 40.35 days
The expression for the radioactivity decay is given as
[tex]N=N_0(\frac{1}{2})^{\frac{T}{t}}[/tex]Substitute the values in the above expression as
[tex]\begin{gathered} \text{0}.25=N_0(\frac{1}{2})^{\frac{40.35}{8.07}} \\ N_0=8\text{ g} \end{gathered}[/tex]