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Red ball... Red ball (Posted on 2002-10-17) Difficulty: 4 of 5
If you take two balls randomly out of a jar of colored balls, there is a 50% chance that the balls will both be red.

What is the total percentage of red balls in the jar?

  Submitted by levik    
Rating: 3.1538 (13 votes)
Solution: (Hide)
This turns out to be a more difficult (and somewhat deceptive) problem than I first thought.

The answer is that 75% of the balls are red. This is because the only viable solution seems to be when there are a total of 4 balls, and 3 of them are red. (You cannot get the same results with 8 balls 6 of which are red, etc)

TomM has submitted a fairly rigorous proof to this in the problem's comments.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
Solutionre: Another solution (re: a mathematecally more rigorous solution)Ady TZIDON2015-08-22 03:34:06
Some ThoughtsNot everything appears as it seemsDej Mar2011-09-13 03:24:48
I do not agree with the official solutionajosin2005-03-04 19:53:25
Some ThoughtsComment on official solutionNick Hobson2004-09-18 18:20:52
Solutionre: solution - and my own onesassy2004-05-13 19:36:19
SolutioneasyBilly Bob2004-03-28 10:22:38
Red ball... Red ballPaula Bilton2003-08-14 08:16:42
(easy) SolutionChaz2003-05-03 14:41:37
Another solution (re: a mathematecally more rigorous solution)Charlie2003-02-17 14:36:06
SolutionsolutionKeedom2002-11-18 23:43:54
AnswerDulanjana2002-10-18 17:11:03
is this right?hai2002-10-18 17:02:51
Solutiona mathematecally more rigorous solutionTomM2002-10-18 02:46:53
typoTomM2002-10-18 00:07:54
SolutionExactly!TomM2002-10-18 00:01:45
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