Very simple card trick, thanks to futilitycloset.com
"Arrange cards with values ace through 9 in a row, in counting order, with the ace on the left.
Take up a card from one end of the row — left or right, your choice.
Do this twice more, each time taking up the leftmost or the rightmost card in the remaining row.
When you have three cards, add their values and divide the total by six. Count the cards that remain on the table from left to right.
The card in the nth position will be the 4."
Is there is a generalization to other numbers of cards?