So, if you know the radioactive isotope found in a substance and the isotope's half-life, you can calculate the age of the substance. Well, a simple explanation is that it is the time required for a quantity to fall to half of its starting value.
These substances are absorbed by our bodies, into our tissues, organs, and bones, and are constantly replenished by ingestion and inhalation.
The last figure I heard was that there are currently eight nuclear subs on our ocean floors. It doesn't work for sea creatures and other things that are under water. Then they measure how much is left in the specimen when they find it.
It comes from cosmic rays that rain down on the earth (and us) from outer space.
In this equation, the units of measure for N and No can be in grams, atoms, or moles.
It does not matter as long as they are like measures.