Interview Question: Light Bulbs

Sample Question #260 (brainteaser)
[By popular demand, another brainteaser. This one was supposedly one of those old Microsoft favorites…]
There are three identical-looking light bulbs in a window-less room (with opaque walls, of course). Outside the room next to the closed door are three unlabeled switches, each controling one of the three lights. You’re told that nobody knows which switch controls which light, but you know that initially all the lights are turned off.
You can flip each of the three switches one and off just once, and after you’re done flipping the switches you can open the door and go into the room once. (That is, you cannot come back out and flip the switches again.) How can you tell which light switch controls which light bulb?
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3 Responses to Interview Question: Light Bulbs

  1. Brett says:

    I personally think this is really a stupid question, with an even more stupid answer.
    What you do is flip the first switch and leave it "on" for a minute. Then flip the second switch and leave it "on" for 15 seconds. Turn both switches off. Go into the room and touch the light bulbs (of course you need to assume you don’t mind getting your fingers burned). You know the rest.

  2. Richard says:

    Brett,I have seen this question elsewhere and it comes with a slightly better answer I guess:Instead of judging by the difference in temperature, which requires more pain of being burned:), one can leave a switch off, a switch on and for the other switch, turn it on for a minute and turn it off.  So when you go into the room afterwards, one is on, and two are off.  Among the two, the one still feels warm is the one you turned on and off.

  3. Brett says:

    Hi, Richard, yes, that’s a better answer!  🙂
    Still, I think this is a dumb question to begin with.  😀

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