The Lightswitch Brain Teaser

[bryand]

This one was apparently on a job application for Microsoft some years ago. You gotta think outside the box. You'll know instinctively once you figure out the only correct answer. If you already know the answer please give others a chance to think about it before posting the answer. Thanks.

You're alone in a building that contains only 2 rooms separated by a hall. The rooms are soundproof and lightproof (i.e neither sound nor light can be heard/seen from one room to the other). One room contains 3 light switches that operate 3 light bulbs in the other room.

You can make one round trip only (starting in either room, going to the other room and then back to the room you started). Assume that (i) the door to each room closes behind you when you leave and (ii) there is enough light in each room to see the switches or bulbs, whichever the case may be.

The task is to label each switch with the corresponding light.

 

[just play]

Oh ya I've seen this before, I forget the answer, and no way could I figure it out.

If I remember correctly it was quite easy when you heard the answer.

 

[Jasminebed]

Don't worry about me, I'll just sit here alone, in the dark.

 

[casinojack]

This one was apparently on a job application for Microsoft some years ago. You gotta think outside the box. You'll know instinctively once you figure out the only correct answer. If you already know the answer please give others a chance to think about it before posting the answer. Thanks.

You're alone in a building that contains only 2 rooms separated by a hall. The rooms are soundproof and lightproof (i.e neither sound nor light can be heard/seen from one room to the other). One room contains 3 light switches that operate 3 light bulbs in the other room.

You can make one round trip only (starting in either room, going to the other room and then back to the room you started). Assume that (i) the door to each room closes behind you when you leave and (ii) there is enough light in each room to see the switches or bulbs, whichever the case may be.

The task is to label each switch with the corresponding light.

When you start, do you know if the bulbs are on or off? Also, are the positions (up being on and down being off) standard ?

 

[bryand]

When you start, do you know if the bulbs are on or off? Also, are the positions (up being on and down being off) standard ?

Great question. At the beginning, all 3 switches are in the down/off position and all 3 bulbs are unlit. All wiring is normal, i.e. switch up = on.

 

[skiny]

You flip on light A and B and wait for 10 minutes. Then you turn off B and go to the light room.

A is on, B is bloody hot when you touch it and C is cold.

I don't need to go back to the other room.

 

[bryand]

You flip on light A and B and wait for 10 minutes. Then you turn off B and go to the light room.

A is on, B is bloody hot when you touch it and C is cold.

I don't need to go back to the other room.

Correct answer. But a round trip is necessary because the task was defined as labeling the switches.

 

[skiny]

Correct answer. But a round trip is necessary because the task was defined as labeling the switches.

I know. As soon as I closed IE I thought... "jeeze I don't follow instructions very well" and came back.

 

[RobWin]

How do you know for sure that a 4-Way light switch was not being used to control all three lights...

Just kidding...
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