Sometimes I nerd-snipe myself with things that are suspiciously close to word problems.

To put into word problem form the question I just nerd-sniped myself with:

If you have a cylinder of diameter 2.5cm and height 10cm composed of pure cobalt in some proportion of isotopes Co-59 and Co-60 that decays at a rate of 3540 Ci, what is the proportion, by mass, of Co-60 in the cylinder? Assume that the density of Co-60 is the same as that of Co-59.

I just wanted to estimate how much of this rod was actually Co-60 (at time of engraving), at least by mass. I'd previously calculated that it was only ~1 Ci remaining, though the exact number will be lower than when I last calculated it, and that measures decays per second, not absolute mass.