Lightbulbs and many other devices break the current path when they fail. Hooking them up in series is a bad idea.
Sometimes a failure produces a low resistance path for the current --- a short circuit. For example, electric tea kettles often fail because the cord is bent back and forth until the insulation wears out and the two wires in the cord make direct contact.
Devices which fail only in this way should be connected in series. However such devices are not common.
In this case all of the voltage drops are the same and they must add up to 120V.
Two light bulbs are connected in series to a 120V supply. The voltage drop across one is 100V. What is the voltage drop across the other?
Since they must add up to 120V, the other voltage drop must be 120V - 100V = 20V.
Just add the resistances:
The same current flows through all and is given by