Given data
*The half-life of iodine-131 is t = 8.07 days
*The mass left is N = 0.25 g
*The number of days is T = 45.7 days
The expression for the radioactive decay equation 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} 0.25=N_0(\frac{1}{2})^{\frac{45.7}{8.07}} \\ N=12.66 \end{gathered}[/tex]