1 lightbulb and 100 prisoners


1 lightbulb and 100 prisoners illustration
1) 1 light-bulb and 100 prisoners***
Once upon a time there were 100 prisoners, all confined in solitary cells. At the center of the prison is a strip of grass with a small cottage with a light-bulb.

One day the evil Nazi commander of the prison camp calls the prisoners together. He tells them:
“Once a day a random prisoner will be allowed to breath fresh air on this strip of grass in the center of our beloved prison. From this the lucky prisoner of the day has access to a small cottage with a light-bulb, which he can switch on or off.
When one of you thinks you have all been here at least once, and states so correctly, you will all receive free library access. If the prisoner is incorrect, you will all end up before the fire squadron.
You now have 1 evening to discuss a strategy, and then its: back to your solitary cells, and no longer any contact.”

What is a good strategy for the prisoners to decide upon?

This puzzle was first presented to me by Jon Koeter, whom I mentioned before. My fellow consultant Harrie Jans surprised me this week by solving it in 1 minute.

You can check your solution at no 210

Did you know?
Showing a brain image besides text makes people believe the text

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2 thoughts on “1 lightbulb and 100 prisoners

  1. It is not necessary that the light is off at the first incidence.
    The best strategy/solution in that case will be they decide that whoever goes there first is their leader when the game starts day after strategy evening. And all others who doesn’t go on the first day themselves know that they are not the leader.

    So, day one if the light is off, the prisoner who goes inside and becomes “leader” by default and turns ON the bulb, but if it is already ON then he does not do any thing.

    Next day onwards.. ..another prisoner go and switches it OFF if he is in the cottage for the first time but does not do anything if he is there for multiple times or if he sees bulb is already switched OFF.

    When leader goes there again and if he sees it OFF then he increments his counts and turns it ON else he also does not do anything. Same goes till his counts become 99 (and himself) 🙂

  2. Small correction in my wordings from above solution:

    It is not necessary that the light is off at the first incidence.
    The best strategy/solution in that case will be they decide that whoever goes there first is their leader when the game starts day after strategy evening. And all others who doesn’t go on the first day themselves know that they are not the leader.

    So, day one if the light is off, the prisoner who goes inside and becomes “leader” by default and turns ON the bulb, but if it is already ON then he does not do any thing.

    Next day onwards.. ..another prisoner go and see if light is on then he switches it OFF and does not do anything if it is already OFF. But he does not do any thing during his multiple visits if he has already switched off the bulb once in one of his visits.

    When leader goes there again and if he sees it OFF then he increments his counts and turns it ON else he also does not do anything. Same goes till his counts become 99 (and himself)

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