This question was put across in an interview for an IT firm

So the interview started with a warm handshake.

Interviewer: So Eshaan, how much would rate yourself on a scale of 10 in terms of your reasoning power?

Me: Sir, I would fairly rate myself 7.5.

Interviewer: That surely is a good score. Actually there is a question that is bothering me from quite sometime now. I was hoping if you could help.

Me: Sure sir ( *smiling as I say that, but inside, his way of framing up the question was making me nervous )

Interviewer: Okay, so there is one room which is totally dark.

It has a bulb holder and a working 100 watt bulb already installed.

The room is closed from all the sides without a window or any ventilator. Except that it has a door without any breathing space below. That means no light from the room can be seen from the outside of the door.

There are three one way switches outside that door. But the bulb inside corresponds to only one of the three switches.

Now suppose that you are on the outside of that room.

The task is to find out that particular switch which controls that bulb.

But you are allowed to enter the room only once.

So tell me Eshaan, how would you solve this problem with your logical reasoning.

(Everything went quiet for several minutes after his constant dictation of that question. I remember I went blank for a moment. I was doing nothing but analyzing all the points. And then he spoke again.)

Interviewer: It’s okay Eshaan. Take your time. But then just keep in mind that you are taking my time as well. ( He was so damn sharp).

*I smiled at this. And repeated all the points of his question briefly. Just to buy myself some time.

And it worked. I had the solution by the time I finished.

I started as follows:

Me: Sir, in my knowledge there could be only one solution to this problem.

I’ll turn the first switch ON and wait for a few minutes before switching it OFF. Turn the switch OFF.

Now I’ll turn the second switch ON.

I’ll let the switch ON and will enter the room.

My observations will be as follows:

There would be two events, either the bulb would glow or it would not.

If I found the bulb glowing, we simply have our answer, it’s the same switch which we just turned ON.

But if the bulb is not glowing I would touch it.

If it is warm, the switch I turned ON for the first time is our answer and if it is at a normal temperature, the switch we did not use at all is our answer.

This way, we have three different results for the three switches.

He was visibly impressed by the answer. It was the first time he smiled and agreed.

“Impressive”, he said.