The "cascade" of isotopes which is produced in the radioactive series is determined by all the intermediate stages.

The visual represents a comparison with water tanks, with different sized outlets and  arranged in series.

Equilibrium of the flow is produced when the same volume flows from each tank.

The volume of water which flows is equal to the activity at a stage.

The diameter of the outlets is the parameter equivalent to the radioactive constant.

The height of the water in the tanks is equivalent to the number of atoms in each isotope at the different stages.

The activity at each stage of the series is:

A equilibrium = l . N