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Probabilistic Card Trick
The
so-called easiest card trick in the world is the following:
Take
a pack of 52 cards. Ask a person to name any two cards – just
the
values not the suits (e.g. ‘Queen’ and
‘King’
or ‘five’ and ‘nine’). Now get
the person to
shuffle the pack and tell them that, miraculously, their chosen cards
will appear consecutively. Say a few magic words and lay the cards out.
It is likely that the two named cards will indeed appear consecutively.
It has been suggested
that the
probability the trick ‘works’ is about 0.9. I do
not know
where that figure comes from, but it is less than this (although maybe
still high enough to do the trick).
I am going to post a
correct solution to this soon. This 'solution'
originally appeared here, but it is wrong (there is some
'double counting' of winning permuations)
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Norman
Fenton
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Making Sense of Probability: Fallacies, Myths and Puzzles