[填空题]
The rubidium isotopek)l489nj)n6bxeg bzwvr b+/ j m0 muwn8o9w/hr95w-n8v vzs (g zjup. $_{37}^{87}\text{Rb}$, a $\beta^-$ emitter with a half-life of $4.75 \times 10^{10}\ \text{yr}$, is used to determine the age of rocks and fossils. Rocks containing fossils of ancient animals contain a ratio of $_{38}^{87}\text{Sr}$ to $_{37}^{87}\text{Rb}$ of 0.0160. Assuming that there was no $_{38}^{87}\text{Sr}$ present when the rocks were formed, estimate the age of these fossils.
$\Delta t$ = $\times 10^{9}\ \text{yr}$
