Well it is very easy to work out the amount of the first radioisotope in a simple decay chain such as P-32 decaying to S-32
For the a unstable daughter such as in Mo-99 to Tc-99m to Tc-99, it is quite easy to work out the Tc-99m activity.
For the second radioactive daughter such as Tc-99 it is quite hard (big equation) to work out the activity at a given time.
What about for a third radioactive daughter such as U-234 in the following chain, where we start with pure U-238.
U-238 to Th-234 to Pa-234 to U-234