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Light Test...

05/04/2010 5:01 AM

OK. If you have already heard this one, don't blow it for the others.

In a room there are three lights. The room is closed and you are outside of the room. In front of you are three switches. Each switch controls one of the lights in the room.

How can you tell which switch controls which light by entering the room ONLY once?

See diagram...

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#1

Re: Light Test...

05/04/2010 5:46 AM

Shout for Mrs Cat to lend a hand.
Del

(Ooooh me sir, me sir I just figured it...but I won't tell)

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#2

Re: Light Test...

05/04/2010 7:10 AM

What silly (ba)nana of an electrician installed that?

  • Does leaving the door open and looking in from outside count?
  • Does knocking the wall and door down so that the switches are in the same room count?

(Wanders off to lay down quietly in a darkened room.... )

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#3

Re: Light Test...

05/04/2010 7:21 AM

Video camera

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#4

Re: Light Test...

05/04/2010 7:48 AM

Assuming all of the bulbs are good:

Number the switches 1, 2 & 3.

turn switch #1 on and wait 10 or so minutes.

turn switch #1 off and #2 on and go into the room.

The hot lamp that's out will be switch #1, the one that's on will be #2 and the cold bulb will be switch #3.

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Guru

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#5
In reply to #4

Re: Light Test...

05/04/2010 9:08 AM

wow good thinking, i would have just drilled a hole in the wall

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#6
In reply to #4

Re: Light Test...

05/04/2010 10:58 PM

Very good thinking! That is the correct answer!

A friend of mine is a facilities guy. When he was hired, they used this question as part of the interview. He shared it with me, and I immediately thought CR 4 would enjoy it.

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#8
In reply to #6

Re: Light Test...

05/05/2010 3:14 AM

Ah, but that's only true if you know the switches are wired with up is off or are labelled off/on . (Which the question doesn't specify)
Del

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#9
In reply to #8

Re: Light Test...

05/05/2010 4:29 AM

Switches are labled: "UP YOURS."

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#11
In reply to #9

Re: Light Test...

05/05/2010 6:11 AM

Ha...I've now captured the moral high ground, from whence I can be smug to greater effect
Del (Nyah, nyah, nyah, nyah nyah)

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#13
In reply to #11

Re: Light Test...

05/06/2010 11:48 PM

Drat! That wily cat has out-thunk me!!!

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#10
In reply to #8

Re: Light Test...

05/05/2010 5:15 AM

Do the switches down under work up = open and down = closed; and if so, why are you following this upside-down convention? Oops, I need to ask some Ozzies and Boers instead. Of course, I recognize that this is all about making the toilet bowl vortices rotate in the correct direction to satisfy Coriolis, whoever that was, but I am willing to descend into abject confusion over momentous questions like this. After all, does your refrigerator light really go off when you shut the door, does a falling tree make a sound if no one is there to hear it, and does a bear...? Inquiring minds want to know about these deep philosophical questions.... Aristotle would be proud/ashamed...

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#12
In reply to #6

Re: Light Test...

05/06/2010 9:22 AM

And it was there in the CR4 challenges a couple of months back, only it was in a shopping stores.

No body remembered .

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#7

Re: Light Test...

05/05/2010 2:04 AM

On the other hand, walking into the room three times would take about 30 seconds, << 10 minutes. The question's answer is admittedly ingenious, and deserves credit. However, failure to "get it" is not much of a discredit, so I doubt its value in an interview. (Not that as an interviewee I would mind, because I knew this. Similar techniques are also useful for checking fan function and direction, for instance.)

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#14

Re: Light Test...

05/11/2010 9:40 AM

G'day TORNADO, yes down under in OZ the light switches are down for closed/on. We don't have any trouble knowing when a tree falls because the mourning cries of the drop bears lamenting the loss of their home/launch pad is heard for many kilometers.

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#15

Re: Light Test...

05/13/2010 5:46 AM

You don't visit the Challenge Questions, do you ? Bwahhh. What kind of lights, are they candles (yeah, the switch operated kind )

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