The number of disintegrations per year of \(15.3\,\text{mg}\) sample of \({}^{60}_{27}\mathrm{Co}\) is
\[
(\text{Mean life of }{}^{60}_{27}\mathrm{Co}\text{ is }7.65\text{ years and Avogadro's number }=6.024\times10^{23}\text{ mol}^{-1})
\]
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For radioactive decay,
\[
\boxed{
A=\lambda N=\frac{N}{\tau},
}
\]
where \(\tau\) is the mean life.