About 92 days are taken for 90 % of the material to decay.
The mass of radioisotopes ([tex]m[/tex]), measured in milligrams, decreases exponentially in time ([tex]t[/tex]), measured in days. The model that represents such decrease is described below:
[tex]m(t) = m_{o}\cdot e^{-\frac{t}{\tau} }[/tex] (1)
Where:
In addition, the time constant is defined in terms of half-life ([tex]t_{1/2}[/tex]), in days:
[tex]\tau = \frac{t_{1/2}}{\ln 2}[/tex] (2)
If we know that [tex]m_{o} = 85\,mg[/tex], [tex]t_{1/2} = 27.7\,d[/tex] and [tex]m(t) = 8.5\,mg[/tex], then the time required for decaying is:
[tex]\tau = \frac{t_{1/2}}{\ln 2}[/tex]
[tex]\tau = \frac{27.7\,d}{\ln 2}[/tex]
[tex]\tau \approx 39.963\,d[/tex]
[tex]t = -\tau \cdot \ln \frac{m(t)}{m_{o}}[/tex]
[tex]t = -(39.963\,d)\cdot \ln \frac{8.5\,mg}{85\,mg}[/tex]
[tex]t\approx 92.018\,d[/tex]
About 92 days are taken for 90 % of the material to decay.
We kindly invite to check this question on half-life: https://brainly.com/question/24710827
