 
When you show this card trick to your group, everyone will want to learn how to do it. Once people learn the trick, they are very motivated to figure out how it works! Learning and practicing this trick involves basic arithmetic skills. Figuring out how it works makes people think about "place value"—that is, how the value of each digit in a number depends on its position.
These instructions tell you how to do a cool card trick that's guaranteed to make you look like a mind reader. Try it with a friend, then spring it on some unsuspecting victims—like your parents!
Materials
A deck of cards
A friend
A pencil and paper
Step 1
Shuffle the deck. That way your victim—oops, your friend—knows you haven't done anything weird to the deck of cards.
Step 2
Ask your friend to choose any card from 1 (ace) to 9, but not a 10, jack, queen, or king. Tell them not to show it to you, but to memorize the number on it.
Step 3
Tell your friend to put his or her card on the table, face down.
Step 4
Now you take a card from the shuffled deck. Choose a card with a number from 1 (ace) to 9. Don't choose a 10, a jack, a queen, or a king. Don't show the card to your friend.
Step 5
Memorize the number on the card, then lay the card face down on the table on the right side of your friend’s card. Be sure that your card is to the right of your friend’s card.
Step 6
Now comes the "magic" part. Give your friend the paper and pencil so that he or she can do some math. Now tell your friend to do the following things—without showing you the answers:
What you tell your friend to do: Example:
Suppose they have the 3 of hearts:
Double the value of the card. 3 * 2 = 6
Add 2 to that number. 6 + 2 = 8
Multiply the result by 5. 8 * 5 = 40
Step 7
So far, your friend has been doing all the math. In this step, you have to do a little math in your head. Take the number of the card you chose. (You remember what it is, don't you?) Then do the following:
What you do: Example:
Suppose your number is 8
Subtract the number of your card from 10. 10 - 8 = 2
Step 8
Now it’s your friend’s turn again.
What you tell your friend to do: Example
Tell your friend your answer from Step 7.
Have your friend subtract that from the number he or she got after doing all the math in Step 6. 40 - 2 = 38
Ask your friend to tell you what number he or she got after doing all that math. Our friend said 38
Step 9
Here’s the magic part. Flip over your friend's card on the table, and then your own. There will be the two digits of the number your friend said. Enjoy the shocked look on your friend's face!
Step 10
But wait! Suppose your friend came up with a different number. Mutter something like, "I know the magic worked—so let’s check your math." Check the math—and you will get the number that’s on the cards.
Download Investigation Sheet. (Microsoft Word Document-64KB)
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