Stamp problem

Result: success

Time limit: until I got it

Question:

The Grand Master takes a set of 8 stamps, 4 red and 4 green, known to the logicians, and loosely affixes two to the forehead of each logician so that each logician can see all the other stamps except those 2 in the Grand Master’s pocket and the two on her own forehead. He asks them in turn if they know the colors of their own stamps:

A: “No.”

B: “No.”

C: “No.”

A: “No.”

B: “Yes.”

What color stamps does B have?

Reflection:

After some deliberation I realized that the part to focus on is that B knew the second time he was asked. Upon hearing everyone else’s uncertainty he became confident that he knew the answer.

Answer:

Therefore B had 1 red stamp and 1 green stamp on his forehead.