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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)

  Submitted by art    
Rating: 3.8750 (16 votes)
Solution: (Hide)
The probability is 1/3.

While it may seem like it should be 1/2, this is not so. This problem can be restated as follows:

"When two coins are tossed, what is the probability that they both show tails, given that at least one is showing tails?"

When two coins are thrown, there can be four possible equally likely outcomes: T-T T-H H-T and H-H. Now the H-H possibility is discarded, since at least one of the coins is showing tails. Of the other three, only one outcome yields the desired result, and since they are all equally likely, the probability of getting that outcome is 1/3.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
Solutionre: AnswerCharlie2014-07-06 10:46:13
SolutionAnswerMath Man2013-01-05 13:54:19
re(5): Question On The SolutionMatt2007-10-25 05:43:18
Some Thoughtsre(4): Question On The SolutionDej Mar2007-10-25 00:58:34
re(3): Question On The SolutionMatt2007-10-24 12:41:58
Solutionre(2): Question On The SolutionCharlie2007-10-24 10:38:57
re: Question On The SolutionCharlie2007-10-24 10:11:44
Question On The SolutionMatt2007-10-23 13:45:31
re: Hehjduval2007-06-13 20:32:53
It COMPLETELY dependsDerek Hyatt2007-03-20 10:33:55
answerK Sengupta2007-03-08 12:42:08
SolutionSolutionowl2005-08-17 14:54:42
AmbigiousKen Haley2004-11-05 01:02:36
Solutionmy answeromar2004-02-21 15:33:52
re(5): Solution Using Real Prob/StatsSilverKnight2003-12-04 07:23:49
re: devil's advocateSilverKnight2003-12-04 07:18:32
devil's advocateLee2003-12-03 23:50:28
re(4): Solution Using Real Prob/StatsCharlie2003-12-03 22:00:28
re(4): Solution Using Real Prob/StatsSilverKnight2003-12-03 16:10:30
re(3): Solution Using Real Prob/StatsSilverKnight2003-12-03 15:43:16
re(2): Solution Using Real Prob/StatsEberhard2003-12-03 14:20:58
re: Solution Using Real Prob/StatsSilverKnight2003-11-26 12:57:24
Solution Using Real Prob/StatsEberhard2003-11-26 12:50:30
re: Actual SoluionRoyCook2003-10-09 16:07:00
Actual SoluionDrew2003-10-09 00:14:58
weakNathan2003-09-05 17:47:18
re: People! Get a clue!Charlie2003-08-03 07:10:11
People! Get a clue!nikki2003-06-25 12:18:47
re(3): I think the solution is WRONGCharlie2003-05-31 08:50:14
re(2): I think the solution is WRONGCory Taylor2003-05-30 09:53:19
Solutionre: I think the solution is WRONGLewis2003-05-30 06:54:40
SolutionI think the solution is WRONGLewis2003-05-30 06:50:41
My first answermark hartman2003-05-08 19:27:02
SolutionI think that...Lewis2003-05-04 06:47:22
re(3): It's not 1/3 the way the problem was posed.Charlie2003-04-11 08:57:28
re(2): It's not 1/3 the way the problem was posed.Charlie2003-04-11 08:54:30
Some Thoughtsre(2): It's not 1/3 the way the problem was posed.Jon2003-04-11 08:19:44
re: It's not 1/3 the way the problem was posed.Bryan2003-03-26 12:02:29
i totally agree with charliejohnson2003-03-14 19:33:56
It's not 1/3 the way the problem was posed.Charlie2003-02-21 08:40:30
MikeTomM2002-05-10 23:11:08
Dummies all aroundMike2002-05-10 16:03:04
Hehlevik2002-04-25 02:47:25
Oh reallyD Willson2002-04-23 22:17:06
Isn't it...levik2002-04-09 10:41:02
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