Mental Agility Plus 1
This mathematical trick can be performed with any pack
Effect: The magician can reveal a chosen card even when it is selected with his back turned.
Method: Whilst your back is turned, get a spectator to shuffle a pack of cards which can be his own if you like, then to select a card from the pack and place it face down on the table. Next, ask the spectator to make two small piles of cards each with the same even number next to his selected card (for example two piles
of four cards or two piles of five cards) and to place one of the piles in his pocket and the other one on top of the selected card which is face down on the table. Finally ask the spectator to take the a pile of cards on the table including his selected card and placed the pile on top of the pack. After all this is done you can turn around.
Now you, the magician pick up the pack and put it behind your back reminding the spectator how you couldn't possibly know which card he selected and its position in the pack. Next count 15 cards from the top reversing their order as you do so and replace them on top of the pack. Bring the pack round the front until the spectator that you are now going to make the situation even more difficult for yourself by asking him to take the small packet of cards from his pocket and place them on top of the pack also. Get the spectator to do this and when it is done, the selected card will now be the 15th card from the top of the pack and of course there are many ways that you can reveal this in as entertaining way as possible.
How the Trick is Done
This is a self working mathematical trick. In fact you could have reversed any number of cards on top of the pack but whichever number it must always the higher than the number contained in each of the piles of the spectator dealt.