3 Light Switches for 1 Light Bulb

You have a set of 3 light switches outside a closed door. One of them controls the incandescent light bulb inside the room. With the door closed from outside the room, you can turn the light switches on or off as many times as you would like.

You can go into the room – one time only – to see the light. You cannot see whether the light is on or off from outside the room, nor can you change the light switches while inside the room.

No one else is in the room to help you. The room has no windows.

Based on the information above, how would you determine which of the three light switches controls the light inside the room?

Answer

Let’s name the switches S1, S2 and S3.

Turn on switch S1. After 5 minutes, turn off switch S1 and turn on switch S2. Switch S3 is never turned on.

Immediately enter the room. If the light bulb is on, it is controlled by switch S2. Touch the light bulb and if it’s warm, it is controlled by switch S1. If it’s not warm, it is controlled by switch S3.

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