Have a look at the link. It's just algebra.
I think your problem is that you may need to begin a 'boringly long way' back in the process. The relationship between rate of emission from a sample and probability of a single nucleus decaying is, I would say, pretty sophisticated stuff - when approached without the back knowledge. But you can always just accept it, in the same way that you (and most of the rest of us) probably accept things like the Cosmological 'facts' we get bombarded with - such as how far it is to the nearest Galaxy and the age of the Universe.where should I begin
Hi dmehling,For the record, I am actually skeptical of deep time geology .