There are 3 switches outside a Room in which one of them are functional to a light bulb which is inside that room.How can you find the right switch if you only allowed into the room once ?

Answer:

Here is the simplification how it works,

First switch on the 1st switch and wait for 5 minutes. Then switch it off and turn on 2nd switch. Now get in the room Immediately and Observe the light bulb.

If it is 'on', then 2nd switch is right.

If it is 'Off', check the bulb if it is Hot or not. If it is 'Hot', then 1st switch is right(because you switched it on for 5 min and bulb gets heated that time) and if 'Cold', then 3rd switch is correct one.