In this problem, we have an exponential decay function of the form
[tex]y=a(1-r)^x[/tex]
where
y is the remaining Bismuth 210 in mg
x is the number of days
a is the initial value
r is the rate of decay
so
we have
a=258 mg
r=13%=13/100=0.13
x=7 days
substitute
[tex]\begin{gathered} y=258(1-0.13)^7 \\ y=97.33\text{ mg} \end{gathered}[/tex]
