After blindfolding them, Paul picks up the five red discs, sticks one on the forehead of each logician, and hides the other discs elsewhere. After removing the blindfolds, everyone sees the discs (all red) on the others' foreheads but not the one on his own.

After a few minutes, one of the logicians (that reasons a little faster than the others) correctly states the color of his disc. How does he work it out?

That's the problem with this puzzle, and some similar ones - at each step in the reasoning for this solution, each logician must know for certain that every other logician has already worked out the reasoning for the previous step, in order to advance to the next. So even the "fastest" logician must wait for the slowest to proceed up to the next-to-last step of the reasoning, and he has no way of knowing when this is (nor who might be slowest). As far as I know, you can't calculate how slow or how fast someone might work out something, because their mental speed might vary from day to day!

Posted by Avin on 2005-09-08 17:04:31