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Simple coins (Posted on 2002-04-09) Difficulty: 2 of 5
I toss two coins and look at the outcome. I then tell you that at least one of the coins is showing up as "tails". What is the chance that the other one is showing "tails" as well?

(from techInterview.org)

See The Solution Submitted by art    
Rating: 3.8750 (16 votes)

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re(2): It's not 1/3 the way the problem was posed. | Comment 10 of 45 |
(In reply to re: It's not 1/3 the way the problem was posed. by Bryan)

You say "Trying to read into the psyche of the puzzle's narrator introduces artificial constraints on the problem, resulting in a distorted answer.", but in order to get the answer of 1/3, one must assume that the narrator, when faced with one head and one tail, would never choose to announce "at least one tail". That in fact is trying to read into the psyche of the narrator and assumes something that was not given in the problem statement. Where does the problem say that when faced with one of each, he'd always report only the head?

A better phrasing of the problem would have been "When asked if there was at least one head, I replied 'yes'," placing the choice to prefer heads upon someone who didn't know the outcome, and explicitly stating in the puzzle a preference for reporting heads.
  Posted by Charlie on 2003-04-11 08:54:30

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