[填空题]
$_{4}^{7}\text{Be}$ decays with a half-life of about 53 d. It is produced in the upper atmosphere, and filters down onto the Earth's surface. If a plant leaf is detected to have $450\ \text{decays/s}$ of $_{4}^{7}\text{Be}$,
(a) how long do we have to wait for the decay rate to drop to 15 per second? $\Delta t$ = $days$
(b) Estimate the initial mass of $_{4}^{7}\text{Be}$ on the leaf. m = $\times 10^{-17}\ \text{kg}$
