Using an exponential function, it is found that the mass of the radioactive sample at the beginning of the 13th day of the experiment was of 169.1 mg.
A decaying exponential function is modeled by:
[tex]A(t) = A(0)(1 - r)^t[/tex]
In which:
At the beginning of the first day of the experiment the mass of the substance was 500 grams and mass was decreasing by 8% per day, hence A(0) = 500, r = 0.08, and the equation is given by:
[tex]A(t) = A(0)(1 - r)^t[/tex]
[tex]A(t) = 500(1 - 0.08)^t[/tex]
[tex]A(t) = 500(0.92)^t[/tex]
At the 13th day, the mass is given by:
[tex]A(13) = 500(0.92)^{13} = 169.1[/tex]
More can be learned about exponential functions at https://brainly.com/question/25537936