Math Is Fun Forum

Math Guy

Member

Registered: 2005-10-12

Posts: 20

The Hall of 20,000 Ceiling Lights

RAY: This next puzzler is from my "ceiling light" series.

Imagine, if you will, that you have a long, long corridor that stretches out as far as the eye can see. In that corridor, attached to the ceiling are lights that are operated with a pull cord.

There are gazillions of them, as far as the eye can see. Let's say there are 20,000 lights in a row.

They're all off. Somebody comes along and pulls on each of the chains, turning on each one of the lights. Another person comes right behind, and pulls the chain on every second light.

TOM: Thereby turning off lights 2, 4, 6, 8 and so on.

RAY: Right. Now, a third person comes along and pulls the cord on every third light. That is, lights number 3, 6, 9, 12, 15, etcetera. Another person comes along and pulls the cord on lights number 4, 8, 12, 16 and so on. Of course, each person is turning on some lights and turning other lights off.

If there are 20,000 lights, at some point someone is going to come skipping along and pull every 20,000th chain.

When that happens, some lights will be on, and some will be off. Can you predict which ones will be on?

*Leave what you think the answer is here please!*

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: The Hall of 20,000 Ceiling Lights

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Why did the vector cross the road?
It wanted to be normal.

Math Guy

Member

Registered: 2005-10-12

Posts: 20

Re: The Hall of 20,000 Ceiling Lights

That's correct, and really quick!

MathsIsFun

Administrator

Registered: 2005-01-21

Posts: 7,711

Re: The Hall of 20,000 Ceiling Lights

Math Guy: More Puzzles, please!

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"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman