When shuffling a deck of cards using a riffle shuffle, one divides the deck in two and lets the two halves riffle down to the table, interleaving as they do so. Assume that a person using this shuffle will always divide a deck of 52 cards exactly evenly, and that the riffle will start equally often from the left as from the right.
The expert dealer that I am, when I perform a riffle shuffle the cards from the two halves always interleave perfectly, the cards alternating from the left and right halves of the deck.
How many times must I shuffle the deck before the probability of correctly guessing the next card down in the deck after seeing a card chosen randomly from some place in the deck will be less than 1.97%? (If the cards were perfectly random, the probability of correctly guessing the next card would be 1/51 = 1.96%)
Bonus: What if there were a 10% chance that, as each card falls during the riffle, the card will be covered by another card from the same half, instead of strictly alternating?
(Assume that the person guessing knows the original order of the cards, the number of times the deck has been shuffled, and the probability of the cards interleaving perfectly.)
(In reply to re: Pascals triangle
Maybe I read to much into the wording, but I see a difference between
having the riffle start from either side with equal probability versus
starting the riffle from both sides equally often. As it is
worded, I would take it that this puzzle is only defined under an even
number of riffles, with exactly half of these (though the order is not
important) left first, the others right first. This will further
duplicate situations and increase the required rifflings.
Further, one must be carefule to realize that the chances of correctly
guessing a following card would not be (necessarily) be the same
foreach of the individual cards, and it is the average over the entire
52 cards deck which must be less than 1.97%.
As if we needed this to be harder.