Suzanne O. answered • 01/04/19
International Experience and Multiple State Certifications
Stamps - Alice Becky Claire
Alice, Becky and Claire are perfectly logical.
Each can make perfect deductions - and each of them knows that the others can too.
(in other words, Becky is analyzing and using the given info from Alice, and Claire is analyzing and using the given info from Becky and Alice)
The three of them are shown seven stamps:
two red stamps, two green stamps and three yellow stamps
They are then blindfolded and one stamp is stuck on the forehead of each girl: the remaining four are hidden away.
The blindfolds are removed and Alice is asked: “Do you know anything - positive or negative - about the color of the stamp on your own forehead?” Alice replies, “No.”
Becky is asked the same question and she also replies, “No”
Claire immediately breaks into a smile - because she knows the color of the stamp on her forehead. What color must it be and why?
Each girl can see the stamps on the foreheads of the other two. When Alice can't determine the color of the stamp on her own head, it can only mean that Becky and Claire do not have the same color stamps, or that they do but they are both yellow. And Becky sees that Alice and Claire do not have the same color stamps or that they do but they are both yellow as well. So it must be that Claire can see that Becky and Alice have the same color stamps, and that the stamps are either red (2 stamps) or green (2 stamps) but NOT yellow (3 stamps). This leaves only yellow for the stamp on Claire's forehead.
Caroline V.
But if Alice and Becky’s stamps are (let’s say) both red, isn’t it possible for Claire stamp to be green?01/05/19