Here’s a slightly more impressive math card trick than yesterday. Start with a full deck of 52 cards [or, rather, start with any 52 cards -- it's not a problem if they come from a mixture of decks]. Give the deck to a Volunteer to shuffle, and afterwards have the Volunteer take the top 12 cards and give you the rest. Sometime during the next step you should sneak a peek at what the bottom card is in the rest-of-the deck. This can best be done by holding your hand casually so the bottom card is face-up enough for you to see, but it looks like you’re just holding them up before placing them down at the end of the next step.
The Volunteer should then look at their 12 cards, not showing them to you, and pick any four of them. [This is a good time to be glancing at the bottom card in the pile you hold in your hand!] Those four get laid face-up in four separate piles on the table, and the poor unchosen eight get put face down in another pile. You put the rest of the deck on top of that pile.
Now think a moment, perhaps idly shuffling the top bunch of cards in the pile [just not the bottom 9], and say that you are going to have the Volunteer lay out some cards and you will predict the last one. Concentrate, and then write the name of the bottom card that you so sneakily looked at before on a slip of paper, fold it before the Volunteer can read it, and give it to the Volunteer for safekeeping. Also give the Volunteer the remaining pile of 48 cards.
Now the Volunteer should look at each of the face-up cards, start with that number, and place cards face-down on each while counting up to 10. For example, if one of the cards is a 7 then the Volunteer would place 3 cards face-down on top, counting “8, 9, 10″. Picture cards count as 10 already, so no cards would be placed on top of those. Be sure that you can still see the original face-up cards.
When all four face-up cards have piles on them, have the Volunteer add the values of the face-up cards. For example, if the cards were a 7, a King, an ace, and a 4 then the total would be 7+10+1+4=22. The Volunteer should count off that many cards face-down from the top of the remaining deck, and then turn over the very last card (the 22nd card, in the case above). Have the Volunteer then look at the folded paper that you wrote on earlier, and Lo and Behold you’ve predicted that very card! Woo hoo!
Like yesterday’s trick, this one is a variation from card-trick.com. Thanks site!