Question

There are 1000 light bulbs lined up in a row in a long room. Each bulb has its own switch and is currently switched off. The room has an entry door and an exit door. There are 1000 people lined up outside the entry door. Each bulb is numbered consecutively from 1 to 1000 and so is each person.
Person No. 1 enters the room, switches on every bulb, and exits. Person No. 2 enters and flips the switch on every second bulb (turning off bulbs 2, 4, 6...). Person No. 3 enters and flips the switch on every third bulb (changing the state on bulbs 3, 6, 9...). This continues until all 1000 people have passed through the room.
How many light bulbs are illuminated after the 1000th person has passed through the room?

• A. 50
• B. 40
• C. 31
• D. 25

Answer 1

The correct option is C 31

Option C is the correct answer. Check the video for the approach.