Red and Blue Hats
Date: 03/20/2001 at 20:37:09 From: Belvona Johnson Subject: Logic problem Here is the question (from page 205 in the Prentice Hall Geometry book): Alan, Ben, and Cal are seated as shown with their eyes closed. Three hats are placed on their heads from a box that contains three red and two blue hats. They open their eyes and look forward. Alan says, "I cannot deduce what color hat I'm wearing." Hearing that, Ben says, "I cannot deduce what color hat I'm wearing, either." Cal then says, "I know what color I'm wearing!" How does Cal know what color his hat is? Here is what I know. If Alan does not know what color hat he is wearing, then Ben and Cal can't both be wearing blue hats, so they could either both be wearing red hats, or one blue and one red. If Ben can't decide either, then I think Cal's hat is most likely red, but I am not sure. Can you give me any hints, or clues to help me solve the guestion? Thank you, Belvona Johnson
Date: 03/21/2001 at 15:09:40 From: Doctor Rob Subject: Re: Logic problem Thanks for writing to Ask Dr. Math, Belvona. You made a good start by realizing that Ben and Cal cannot both have blue hats. The possibilities after Alan's statement are: Alan Ben Cal B B R B R B B R R R B R R R B R R R Now if Ben can see that Cal's hat is blue, he can deduce that his hat is red, because Ben and Cal cannot both have blue hats. Since Cal cannot deduce this either, the second and fifth possibilities above are out. That leaves this: Alan Ben Cal B B R B R R R B R R R R You can see that in every remaining case, Cal is wearing a red hat. - Doctor Rob, The Math Forum http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.