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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?

See The Solution Submitted by levik    
Rating: 3.1538 (13 votes)

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Some Thoughts Comment on official solution | Comment 12 of 15 |
As Charlie pointed out, there is no unique percentage.  3 reds out of 4 balls is one solution; the next smallest is 15 reds out of 21.  This does not yield 75% reds.

The cases in TomM's solution are not broken down correctly.

For example, (r+o)(r+o-1) = 2r(r-1), with r and o mutually prime, implies r+o *divides* 2(r-1), but not r+o = 2(r-1) or r+o = r-1.

  Posted by Nick Hobson on 2004-09-18 18:20:52
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