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# Riffle Shuffle

## Question

Count a deck of casino poker card face up to show to your audience that all suits are complete.

Restack and riffle shuffle repeatedly until they say enough.

Spread the deck across face down and let anyone take a card.

The card should be shown to the others (except you) to witness.

Ask for that card to be returned face down elsewhere on the spread deck.

Restack and riffle shuffle repeatedly until they ask you to stop.

What is your chance of finding the picked card?

## 10 answers to this question

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Isnt it just 1/n assuming the casino deck is made up of n distinct cards?

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The casino deck is made up of 52 distinct cards. 1/52 is not the maximum.

You can bet for a good odd by this specially when the witnesses examine the picked card carefully..

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Does this have to do with the deck being brought back into its original order (assuming perfect shuffling) after so many shuffles? If you were skilled enough to "follow" a card and you knew where it started, you would have a pretty good chance of finding it.

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Does this have to do with the deck being brought back into its original order (assuming perfect shuffling) after so many shuffles? If you were skilled enough to "follow" a card and you knew where it started, you would have a pretty good chance of finding it.

Negative..the cards randomizes well. T

he no. of shuffling is not controlled..If it is a new deck from a wrapped box (where the suits are supposed to be in order - A-K (Hearts) , A-K(Diamonds) ,A-K(Spades),A-K(Clubs), Jokers removed) it is much easier to follow.

But the deck in OP is not brand new so you are skilled enough ..

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Does this have to do with the deck being brought back into its original order (assuming perfect shuffling) after so many shuffles? If you were skilled enough to "follow" a card and you knew where it started, you would have a pretty good chance of finding it.

Negative..the cards randomizes well. T

he no. of shuffling is not controlled..If it is a new deck from a wrapped box (where the suits are supposed to be in order - A-K (Hearts) , A-K(Diamonds) ,A-K(Spades),A-K(Clubs), Jokers removed) it is much easier to follow.

But the deck in OP is not brand new so you are skilled enough ..

With enough skills and years of practice you can:

1. Memorize the initial positions of individual cards. (When exposing the cards)

2. Cut the deck 26-26 to shuffle one right over one left card all the time.

3. Follow the resulting new position of individual cards every shuffle.

4. Spread the deck of known positions of individual cards and watch

which card was drawn. (not even a talent)

It is the Queen of Diamond!

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With an assistant in the audience who can subtly signal the rank and suit of the chosen card to you, the odds of identifying that card are pretty close to 100%.

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Does this have to do with the deck being brought back into its original order (assuming perfect shuffling) after so many shuffles? If you were skilled enough to "follow" a card and you knew where it started, you would have a pretty good chance of finding it.

Negative..the cards randomizes well. T

he no. of shuffling is not controlled..If it is a new deck from a wrapped box (where the suits are supposed to be in order - A-K (Hearts) , A-K(Diamonds) ,A-K(Spades),A-K(Clubs), Jokers removed) it is much easier to follow.

But the deck in OP is not brand new so you are skilled enough ..

With enough skills and years of practice you can:

1. Memorize the initial positions of individual cards. (When exposing the cards)

2. Cut the deck 26-26 to shuffle one right over one left card all the time.

3. Follow the resulting new position of individual cards every shuffle.

4. Spread the deck of known positions of individual cards and watch

which card was drawn. (not even a talent)

It is the Queen of Diamond!

YES! It is the Queen of Diamond.
I don't think you did 1-4..You just used telephaty.
Let's deal with a more practical and realistic probability.

Edited by TimeSpaceLightForce

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With an assistant in the audience who can subtly signal the rank and suit of the chosen card to you, the odds of identifying that card are pretty close to 100%.

There is no assisted trick here..just plain gambling.

Edited by TimeSpaceLightForce

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The more controlled steps:

1.Riffle shuffle repeatedly until they say enough.

2. Spread the deck radially on hand and let anyone draw a card.

3. Close the deck and ask for that card to be witnessed.

5. Riffle shuffle repeatedly until they ask you to stop..

this will give you about 40% of finding the picked card.

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Some poker cards would look different if turned 180. There are 22 of such (e.g. Ace of heart,of spade ,of clubs)
It is hard to remember all their polarities from random but only about half of them should be pointing north..thus the others pointing south.
If by chance one of these was picked and the deck polarity is always maintained during the riffle shuffes,
then it would be up to the chance of turned picked card when returned that finding it is possible.
This is increased when the deck is spread from right to left.Not sure but observed.
When awkward, the picker should be using the other hand on the card causing it to rotate.Witnesses passing around for checking
can also cause that card to rotate. Anyway,when a card is extracted from the rest it may returned with different polarity.
So, it is 1/52 X(A)X(T)
A =the number of memorized asymmetric cards polarity
T =chance of a turned picked card when returned to deck
For controlled:
Polarize all asymmetric cards to one direction before hand..so no need to expose the cards.
Polarity of deck shall be kept during shuffles.
When the picked card was drawn and shown you are opt to rotate or retain the deck on hand in
order for the picked card to be returned to deck on counter polarity as much as possible.
That then would be 22/52 chance.Favors a 1 bet that pays 2.
If you failed to find it, then the picked card is a symmetric one..with 1/30 chance guess.
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