This mathematical trick can be performed with any pack
Effect: The magician can reveal the exact number of cards taken by a spectator.
Method: The magician asks a spectator to take a few cards from the top of the pack but to conceal them from the magician to the extent that the neither the performer nor the spectator can tell how many were taken.
The magician then also takes a bunch of cards secretly making sure he takes more than the spectator. Next, the magician asks the spectator to turn his back and count his cards silently so as to give no clue to the performer.
The magician counts his own at the same time then in a novel albeit roundabout way reveals the exact number of cards held by the spectator.