I know a couple of math tricks you can do with playing cards.

• Give an audience member a pre-sorted deck of cards. Remove any card. Riffle shuffle the deck 3 times (4 if you're bold). Return the removed card to a random position in the deck. Magician examines the deck and identifies the card.

• Audience member selects any 5 cards and hands them the the magician's beautiful assistant. Assistant examines these cards, and reveals 4 of them, one at a time, to the magician. Magician predicts the 5th card.