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Keep them in that order. The ace will represent 1 for this trick and will be the very top card on your pile. The cards can be any suit or color. Do this before your audience arrives. The rest of the deck can be in any order. Avoid shuffling after you put the cards on top, though, so you don't mess them up. Be careful not to jumble the cards as you put them in your pocket. You want the ace, 2, 4, and 8 to stay on top. Remember which side of the deck is the top when you hide the cards. If you don't have a pocket, hold the cards under a table or anywhere hidden where the audience can't see them. They can choose 15, but it can't be any number larger than that. Have them tell you their number out loud. You can have them write down the number on a piece of paper, too, if you need a visual. Every number between 1 and 15 can be formed by adding up some or all of the 4 top cards in your pocket. The collection of numbers that sum up to the number your audience gave you is known as its binary decomposition. Instead of trying to add different numbers using trial and error, start by subtracting the largest of your 4 cards from the audience's number. Then subtract the next largest number and so on, until you reach 0. Don't use a calculator or paper to do your calculation. You have to do it mentally for the trick to be impressive. You should know where each card is because you set them on the deck in the order of 1 (ace), 2, 4, and 8. Carefully remove the correct cards. For example, if the number is 10, you'll need the 8 and 2. The 2 is the 2nd card in your deck and the 8 is the 4th. If you accidentally pull out the wrong cards, you'll ruin the trick. Once you pull out the cards, arrange them in your hand so that the numbers are visible. Then flip them around so your audience can see them. Explain that the numbers add up to their chosen number. You can also lay the cards out on a table or simply hand the cards to your audience. If you want to repeat the trick, make sure you put the cards back into your pocket in the correct order.

Summary:
Put an ace, 2, 4, and 8 card on the top of the deck. Slide the deck of cards into your pocket. Ask your audience to pick a number between 1 and 15. Calculate in your head how to get their number using 1, 2, 4, and 8. Pull out the cards that add up to that number without looking. Show the cards to your audience.