Answer:
[tex]\large \boxed{4.0}[/tex]
Explanation:
[tex]\text{No. of half-lives} = 2.8 \times 10^{9 } \text{ yr} \times \dfrac{\text{1 half-life}}{7.0 \times 10^{8} \text{ yr}} = \textbf{4.0 half-lives}\\\\\text{The sample went through $\large \boxed{\textbf{4.0 half-lives}}$}[/tex]
