Radium-226 has a half life of 1620 years. Find the time period during which a given mass of this material is reduced by one-quarter.
Q= Ce^-rt should be the right equation?
Stumped on how to set this up.
Because you are given a half life it is better to work with powers of $\displaystyle $$2$ rather than $\displaystyle $$e$. So:
$\displaystyle Q(t)=Q(0)\times 2^{-t/\tau}$
where $\displaystyle $$\tau$ is the half life.
Now if at time $\displaystyle $$t;\ Q(t)=Q(0)/4$ we have:
$\displaystyle 2^{-t/\tau}=1/4$
CB