You are standing outside a room with no windows and a solid door. You cannot see into the room.
On the wall outside the room are 3 switches which are connected to 3 lightbulbs inside the room.
Each switch is connected to one light bulb.
You must set the switches and then enter the room.
When you enter the room you will be able to tell which switch is connected to which bulb.
You can only enter the room once.
The switches are labelled A, B, and C.
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Here is the solution:
You must rely on the fact that lightbulbs generate heat as well as light, so a bulb will remain hot for a while after it is switched off.
Turn switches A and B to On and wait 1 minute.
Before you enter the room turn switch B to Off.
The bulb which is lit is connected to switch A.
The bulb which is unlit but warm is connected to switch B.
The bulb which is unlit but cold is connected to switch C.