I kinda got stuck on this one here.
It says:
Einsteinium (254Es) has a half-life of 270 days. How much of a 5-gram sample would remain after 2160 days?
recall that $\displaystyle r = \frac {\ln 2}{\text{half life}}$
use that $\displaystyle r$ in the exponential decay formula: $\displaystyle N(t) = N_0e^{-rt}$
where $\displaystyle N(t)$ is the amount remaining after time $\displaystyle t$, $\displaystyle N_0$ is the initial amount, $\displaystyle r$ is the rate of decay
you want to find $\displaystyle N(2160)$