The puzzler about 20000 lights seemed to have a setup without an actual problem/puzzle to answer. Can anyone fix it or tell me what it’s supposed to say or explain what the question is?

# 20000 lights Puzzler without a question? Feb 11 2019

**ABLE1**#3

I was going to ask the same question of what was the question??

Seems that **geoffhazel** beat me by 6 minutes.

**geoffhazel**#4

I found it. Rerun of a puzzle from 2011:

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

**eddo**#5

this part was on the new one when I went and checked, so someone must have figured out they left it off.

It’s the puzzler on the most recent podcast, 2.9.19.

https://www.cartalk.com/puzzler/hall-20000-lights

This is how the question ends …

"Now, a third person comes along and pulls the cord on every third light. That is, lights number 3, 6, 9, 12, 15, et cetera. 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?"

**Purebred**#7

Anytime the puzzler is a non-automotive question I just ignore it.

Like if a train leaves Chicago traveling at 30 MPH, how many pancakes does it take to shingle a roof?

This is sort of an interesting puzzler for math-types, which Ray and Tom tend toward. The answer must be closely related to the ancient Eratosthenes Sieve algorithm, so the answer likely has to do with prime numbers. They have a sculpture at nearby (to me) Stanford University, a huge Eratosthenes Sieve, which I enjoy visiting whenever I go there for a walk-a-bout.

**Triedaq**#10

I don’t think it relates to prime numbers. Write out the numbers from 1 to 36 and mark a 1 under the number of each light when it is turned on and 0 each time it is turned off. You will see what is happening.