There are 100 people and 100 lightbulbs. All the lightbulbs are off.

The first person flips the switch and turn all the lightbulbs on.

The 2nd person flips the switch of every 2nd lightbulb.

The 3rd person flips the switch of every 3rd lightbulb.

The 4th person flips the switch of every 4th lightbulb and so on till the 100th person.

How many lightbulbs are on after the 100th person flips the switch?

Do not use Excel to solve the question.

EDIT: Actually, please feel free to use any methods to solve the puzzle. :P Heheheh....I actually was looking to see if there's any other way apart from using Excel, which I did. :P

Sorry, it's the 4th person flips the switch of every 4th lightbulb.