Thirty-five years ago I was shown a simple card “trick” that has been useful at numerous social events. I say “trick” in scary-quotes because there really isn’t a “trick” to it - no misdirection or such. This trick requires no slight-of-hand or even the person performing the trick to even touch the cards (it’s better when you have someone else perform all the steps at your command). It is based on some mathematical logic, yet I don’t know what it is. When I demonstrate the trick, I’m always asked how it works, but I don’t have an answer, simple or complex. I even presented it to my high school algebra teacher*, who also couldn’t (or wouldn’t) provide an explanation. I call upon the forces (dark or light, I don’t care) of the SDMB to provide a relatively simple (for a simple person) explanation.
Summary: predict the top card of a stack of cards based purely on counting other cards.
The actual practice of this trick is much simpler than it appears when describing it. I tried to make this as accurate yet concise as possible, but I’m sure others can find a simpler way of explaining some steps.
Start with a standard 52-card deck (no Jokers). It is CRITICAL that all 52 and only the standard 52 cards are present.
Place a card from the deck face-up on the table, then add additional cards to make a value of 13.
Examples:
A) the first card placed is a “4”; counting from 4, add another card (say “five” out loud for effect), then another, and so forth until you count to 13. In other words, if your first card is a “4”, you’ll add 9 additional cards (10 cards total in the stack).
B) the first card placed is a “Jack”, which counts as “11”, so you would add 2 additional cards for a total of 3 in the stack. Queens count as 12 and Kings count as 13. A “King” stack would only consist of one card. You are free to put the card back in the main deck and choose another card, it won’t matter.
The values of the cards you are adding do not matter; only the value of the first card of each stack (which will become the top card of each stack).
Once you have made a stack of “13”, turn the stack over face-down.
Continue making stacks until you have roughly 5-8 stacks (the minimum number of stacks needed is 3, as we’ll soon see).
Keep all unused cards in the main deck.
Remove all but 3 of the stacks, putting the removed cards in with the main deck (the order you combine them is irrelevant).
Turn over the top cards of 2 of the stacks; add the two numbers together (example: if the two cards were a “six” and a Queen, that would total 18).
Add 10 to the total of the 2 cards (using the example immediately preceding, 18+10=28). Let’s call this number “X”.
From the main deck, count out “X” number of cards (in our example, we’d count out 28 cards).
Then, again from the main deck, count HOW MANY cards remain (not their values, simply how many cards you were left with in the main deck after removing “X” cards).
That number of remaining cards is the value of the top card of the third stack. Turn over the top card of the third stack to ooohs and aaaahs. Or ooze and oz.
This is the essence of the trick. Showmanship can obviously improve presenatation.
All this to ask: what is the basic mathematical scheme by which this works (dumbed down for me, a Criminal Justice major)?
- many, many years ago, obviously