Light Bulb Switching Brain Teaser

[riveradn]

I figured it out in less than 5 minutes!

[starter605]

@alkoed Ha, that is so true. Though only 1 and 100 stay on because its skip counting by 100 no by squares.

[alkoed]

None of them will be on because none of them were screwed in.

[aitm16]

it was soo easy after he explained it!!

[91fatboy51]

I tried this for real: I hurt my fingers on the light switches, my electric bill went up $30 before my wife left me, then my house caught fire due to electical overload and burned to the ground, the neighbors called 911 to report arson, and the local sheriff arrested me as a suspected psycho trying to contact aliens by flashing messages in morse code! To top it all, I used up over 300 light bulbs before the law threw me in the nervous hospital with padded walls and bars on the windows. Help!

[Justinfh2point0]

No wonder why I couldn't figure this brain teaser out, it's math! Regardless, I agree, this should be taught in public schools.

[xxflipflopxx13]

Only the first lightbulb would still be on

[dominic2751997]

those bulbs are going down



[isaiashernandez44]

thanks gret one sal!



[kevi8991]

Great brainteaser! Though not particularly the most difficult one, it's still fun to solve :D Oh yeah, and @crille12300 I think he's drawing with a tablet of some sort, because he's writing really really fast for mouse-based writing.

[FantasticMetro]

Just amazing!

[crille12300]

You are pretty good at drawing with the mouse.

[beno2367]

hmm, they all actually do increase odd numbers though.. didn't know that lol

[CosmosClinicAu]

Solve it in a minute. Interesting video, although - I've seen better from you, Sal...

[SonofOxford]

'Think' and 'see' for the answer; reason + imagination FTW

[freezeme360]

After 100 pusses

[nzvelcom]

Hey, Sal...this was toooooo easy! What happen? ;)

[ethnolink]

Got it right! This is one of my favorites!

[CoolScienceAustralia]

This is cool...and so simple! Good one! :)

[gnomepunter63]

:...

[missypunkrocker]

I did it!

[bryanfuel]

well im off to go buy 100 lightbulbs and 100 switches...

[Ninjoark]

Yeah 0 are on because there aren't any batteries.

[wkdtjrghks]

This is soooo easy. I solved it in one second. O.o All square numbers are switched on because they have odd number of factors.

[foroke1]

Fuck math

[SSDsmiley]

all the lights will be burnt out.

[Artisablast261]

we will be finding the no. of divisors of the no.s

if its even it will be off

else on

[BeastyBoi101Rushezzz]

i thinks it 1

[ghfreakwierdo123]

I figured it out but it was still nice

[goldblend86]

felt good to work out... even though it didnt take long... and you do have a great voice.

[kendroctopus]

Very interesting!!!:)

But, gah, i have to admit i didn't get it!

[jigglopop2]

oooh ligh bulbs so retarted!

[trajendran86]

The other interesting property which you could teach is that the squares of prime numbers have only 3 factors and no more!! Just thought I could point that out over here.

[ff2paladin]

Could have taken this opportunity to give a small calculus lesson. When graphing the perfect squares as a function of x (x=1, y=1; x=2, y=4, x=3, y=9, etc), the difference between the differences (3,5,7,9, etc) (double differential) is 2, which is the double derivative of x^2 with respect to x.

[v8kiu]

Cool Puzzle :)) I think all the light bulbs will be burnt on 100th pass ^^

[nyra007]

I made a java program for this once. so yeah all perfect squares are on after 100 passes.

[boeing747200lr]

I did an excel spreadsheet. Thanks for the hint Sal.

[nejtilsvampe]

as I read the discription "Turning light bulbs on and off." my light in my room turned on and off for a second - spooky xD

[Zeratul723]

That's well done. I figured it out right before he gave ALL the details, but I still needed some coaxing. I've lost my touch, it seems....

[mauroprovatos]

mind is blown

[eebamxela]

I cheated with excel. the results are pretty sweet. if you apply conditional formating using colors representing on and off, and then expand the problem to like 1000 bulbs and 1000 passes it's even cooler.

[mikeysomething]

1:49 then im going to come...

i laughed

[MADxMANxLUKE]

@mikeysomething lol same

[whyld2171]

very good!

[mohawow]

Pure math "stat" question! I need an equation lol

[melese1988]

If N is a perfect square then it has odd number of divisors. Conversely, if a number N has an odd number factors then it's a perfect sqaure.

[wexarexthexStorm]

Thats actually really cool.

[XANDERXXZ]

He has a cool voice.

[isaiashernandez44]

duh

[rolfch1p]

this problem is similar to the math problem involving the locker doors and the students.

[Pet3rGr1ff1n]

FIRST!

[buchanass]

hahaha nice!! a little tedious at first but fucking awesome got my noggin going!

[mario1337]

thanks for your work.

i appriciate it.

[kionay]

hope i enjoyed that?

i did

when i realized the thing about the perfect square right before you said it, i was just amazing

i enjoyed that quite a lot

[jacobromu]

c&o ftw

[thelastwords]

awesome!

[eNathan314]

This guy owns!

[Ren520]

i solved this one like half a yr ago...

its pretty interesting. keep em coming!

[spider853]

Do you know how to solve this problem?

acmp(dot)ru/index.asp?main=tas­k&id_task=337 P.S. you can use google translate for translation.

There is no space in URL

[XANDERXXZ]

Its at the end of the video.

[cwldoc]

Another great video! Thanks for posting.

[kombatwombat16]

Nice vid. I thought the question was a little easy though, since I instantly thought of the Sieve of Eratosthenes. After that I thought about factors and such, but I learned a little interesting tidbit of number theory through the process.

Also, I wrote a python script to solve this question.

[chankpop]

I got it right :-)

Used a different method though.

alternate method: let on =1, off = 0

I did ten passes for the first ten numbers so

pass 1: 1 1 1 1 1 1 1 1 1 1 ---------1

continuing to

pass10: 1 0 0 1 0 0 0 0 1 0 -------1

from there its just a matter of looking at the ones and zeros and coming up with the following solution.

1 (two zeros) 4 (four zeros) 9 (six zeros) etc.

its just a different way of thinking about things.

[EFP055]

that's exactly what I did too!

[doomdart]

enjoy i did

[GuidoPerdomo]

that is cooooooool!

[spider853]

I have one question trough, Its there some quick solve for this kind of problem: you have n light bulbs, each light bulbs will be switched for a given interval k (<n) for z intervals how many light bulbs will be on? for example: n = 5; z = 3; k = [2,3,1] <- 3 numbers as z so the passes: 1. 0 0 0 0 0 2. 0 1 0 1 0 (for 2) 3. 0 1 1 1 0 (for 3) 4. 1 0 0 0 1 (for 1) So here the answer is 2 light bulbs are on. but the problem is that n can be 99999999 so I need I quick solve or some kind of systematic

[spider853]

youtube have some kind of bug and putted some br's instead of enter (newline)

[hendrix5045]

Can noone see the video response that I made? I wrote a program that does this problem in C++. The link is youtube(dot)com /watch?v=0bm5HCDF7-0

[spider853]

I know such a problem on a C++ testing site. Didn't resolve it because of resource limits. 1MB/1sec

[xhotshot12]

awesome love tye prob

[Mattprole]

Simple but genius!

[pmip247]

Awesome man, I love watching these videos!

[hendrix5045]

I just posted a video response but it's not showing up. Did you get anything?

[shootshoot]

No reason to hope I enjoyed it.... I definitely enjoyed that.

[DURound]

You could do another one of these... except only leave then lit if they have exactly two factors.

[siggyboss]

I did enjoy that.

[norwayte]

Patterns are always interesting. Everywhere.

[princeofexcess]

@norwayte its confirmation bias THERE ARE NO PATTERNS THEY ARE ALL FAKE!

[veers0r]

I did only come to the point that it is the ones with an uneven number of divisors, I didn't figure out that those are only the perfect squares. Pretty cool :)

[yuskei3]

In before seeing answer.

10 lights.

All the perfect squares.

[yuskei3]

Got it right =D

[ScotsmanRS]

That was pretty damn interesting! :)