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 This or that? (Posted on 2015-08-19)
Alan and Bob are playing a game of marbles. Alan has two marbles, Bob has one, and each rolls to try to come nearest to a fixed point. If the two have equal skill, what is the chance that Alan will win?

There seem to be two contradictory arguments. On the one hand, each of the three marbles has an equal chance of winning, and two of them belong to Alan, so it seems that there’s a 2/3 chance that Alan will win.

On the other hand, there are four possible outcomes:
(a) both of Alan’s rolls are better than Bob’s,
(b) Alan’s first roll is better than Bob’s, but his second is worse,
(c) Alan’s first roll is worse than Bob’s, but his second is better, and
(d) both of Alan’s rolls are worse than Bob’s.
In 3 of the 4 cases, Alan wins, so it appears that his overall chance of winning is 3/4.

Which argument is correct?

Source: J. Bertrand, Calcul des Probabilités, 1889, via Eugene Northrop, Riddles in Mathematics, 1975.

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 Playing the lottery. | Comment 3 of 4 |
A man buys a lottery ticket and says "The way I figure it there are two possibilities.  I will either win or I will lose. So I have a 50-50 chance of winning."

The fallacy here is the same.  Just because you can enumerate four possibilies that doesn't mean they are equally likely.

 Posted by Jer on 2015-08-19 10:49:33

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