Radioactive nuclei can decay into other radioactive nuclei which can in turn decay. Thus we can have an entire series of related radioactive decays. The analysis of the activity must then take into account both the production and decay of a particular species.
We consider the decay ( 
of a parent nucleus into a daughter
product which
in turn decays ( 
into a granddaughter nucleus, which we will
take to be stable. (this is easily generalised).
Then
Solving these equations,
and
Often we can take 
, i.e. we start with pure
1. Then:
