Using the half-life formula:
\( t = \frac{\ln(N_0/N)}{\ln(2)} \times T_{1/2} \)
where \(N_0 = 10\), \(N = 2.5\), and \(T_{1/2} = 5730\) years.
Substituting the values:
\( t = \frac{\ln(10/2.5)}{\ln(2)} \times 5730 = 11460 \text{ years}. \)
The half-life of a radioactive nucleus is 5 years. The fraction of the original sample that would decay in 15 years is: