A moderator takes a set of 8 stamps, 4 red and 4 green, known to three logicians, and affixes two to the forehead of each logician so that each logician can see all the other stamps except those two in the moderator's pocket and the two on his or her own head. He asks them in turn if they know the colors of their own stamps:
1. A: "No"
2. B: "No"
3. C: "No"
4. A: "No"
5. B: "Yes"
What are the colors of B's stamps?
Given that the problem is symmetric (there are as many red stamps as green ones), if the solution was Red-Red, then it could also be Green-Green, so if someone could reason his colors and reach an unique solution, it must have been RED-GREEN.
No thinking needed
