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Attributed to Lewis Carroll (Posted on 2017-11-07) Difficulty: 2 of 5
A bag contains one counter, known to be either white or black.
A white counter is put in, the bag shaken, and a counter drawn out, which proves to be white.
What is now the chance of drawing a white counter?

No Solution Yet Submitted by Ady TZIDON    
Rating: 3.0000 (1 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
Solution Without replacement (spoiler) | Comment 1 of 4
Assume that the original counter is equally likely to be black or white.  Then there are 3 equally likely cases:

a) The original counter was black, and you have just drawn the new one

b) The original counter was white, and you have just drawn the new one

b) The original counter was white, and you have just drawn the original one

The counter remaining in the bag is therefore white with probability 2/3.

  Posted by Steve Herman on 2017-11-07 11:59:51
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