I want to make sure I am approaching this problem correctly:

Element X has a half-life of 100 years. If I have a 128 Curie sample of element X, how long will I have to wait until its radioactivity has decreased to 0.5 Curies?

I believe I can use the radioactive decay law? Where N(T) = .5 Curies, inital is 128 Curies... etc. I am not sure and the unit of Curies is confusing to me. $$N(T) = N_{0} \cdot e^{-\frac{ln2}{half-life}t}$$

• If you have a $128\,\mathrm{Ci}$ sample of anything you'd better know a lot about radiation protection ... – dmckee Nov 30 '17 at 20:29

The Curie is a less used measure of activity of a radioactive substance $($Personally I'm a lot more familiar with the Bequerel$)$. So, in short, yes you can use that equation to calculate the final activity.
Looking at the units you can see that in the power of $e$ part you essentially have $\frac{t}{t}$ which cancels out, leaving you with just $Ci=Ci$, so it checks out :p.