# Math Help - Radioactive decay

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.

2. Originally Posted by dmbocci
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 $2$ rather than $e$. So:

$Q(t)=Q(0)\times 2^{-t/\tau}$

where $\tau$ is the half life.

Now if at time $t;\ Q(t)=Q(0)/4$ we have:

$2^{-t/\tau}=1/4$

CB