Final answer:
When a bulb in a string of lights wired in series like an open switch burns out, all other bulbs go out; each bulb in a 40-bulb series uses 3 V normally. If a bulb in a newer series wired like a closed switch burns out, remaining bulbs stay on; with 39 bulbs left, each would use approximately 3.08 V.
Step-by-step explanation:
When strings of holiday lights are wired in series and a bulb that functions like an open switch burns out, it breaks the electrical connection, resulting in all other bulbs going out since the circuit is not complete. For a string operating at 120 V with 40 identical bulbs, the normal operating voltage of each bulb is 3 V (120 V divided by 40 bulbs). However, if we consider newer versions where bulbs short circuit acting like a closed switch when they burn out, the rest of the bulbs will remain lit when one burns out. If one bulb burns out from a string that operates on 120 V and originally had 40 bulbs, the remaining 39 identical bulbs would then operate at approximately 3.08 V each (120 V divided by 39 bulbs).