Puzzle n°21 (***) - Light ! Homepage  

A weird story...
In order to test their sense of cooperation, fifty somalis had been brought together
in a remote castle, and they could be freed at one condition only, which I will now explain to you.

In fact, their keeper, a big fat and mean cat named Raminagrobis,
had explained the rule of the game to them upon their arrival, in these words :
"From tomorrow onwards, I will put each of you in a separate room and you will not be able to communicate any more.
I will then choose one of you randomly and bring him or her in the castle central room,
which contains solely a bulb and its switch. This bulb is currently switched off.
During your time in this room, you will be free to leave the bulb as you found it (lit or switched off),
or to change its state (switch it on if it was off, or switch it off if it was lit).
Then I will bring you back to your room and fetch any one of you for the same procedure,
over and over again, because my patience has no limits and I am able to go on forever with this game.

What should I do ?

However, if one of you is capable to tell me, at any time, that he or she is sure that everyone of you
has already visited the lamp room, then I will free you all !
But, should he or she be wrong, then I will execute you all..."
Having said this, he left them all together to discuss the best strategy before being isolated.

The somalis were shivering from fear...
However, after quite a few hours of discussion, they fell asleep soundly,
because they had collectively agreed on how to proceed to be sure to be freed some day.
It might take a long time, but they would manage !

If you had been among them, what strategy could you have proposed to achieve their release ?

... I'm giving up ...


© Marie-Bernadette Pautet, 2005-2019
Last page update: 8.8.2007