There is a light in a room with 3 switches outside.
One of the switch is connected to that light.
You are allowed to enter the room only once.
How will you find which switch is for that light?
Set the first switches on for about 10 min, and then switch on the second switch and then enter the room. Three cases are possible
1.Bulb is on => second switch is the ans
2.Bulb is off and on touching bulb , you will find bulb to be warm
=>1st switch is the ans
3.Bulb is off and on touching second bulb , you will find bulb to be normal(not warm)
=>3rd bulb is the ans
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