Question

Dear Mitch,

I have a student who needs practice doing the kind of arithmetic in his head that he should be able to do by now. The only thing that seems to interest him is being able to perform magic tricks that rely on a little math to do things that seem like magic for his friends. I've heard there are things I could introduce him to that would thrill him and get him to want to learn more math so he'd have more tricks to perform.

I have a vague recollection of a trick I knew as a child with five or six cards that you have a person separate by whether or not a certain number appears on each card. I think that the number has to be secretly selected, but I don't know remember much more than that. Do you have any idea what I'm referring to?

Sincerely,

Mr. Freddy

(A pretty good teacher, but not a magician)

Answer

Mr. Freddy,

Yes, I know exactly the one you're talking about. It is very old, but it's stuck around because it's so much fun. It involves six cards, and the person performing the trick tells another to look over the set of cards carefully, pick a number on one of the cards, and then go through the other cards to see which ones contain that number. Then the participant separates the cards that show the number he selected and sets them down while handing the others back to the 'magician'.

In a few short moments, the mathematical magician glances at the cards and -- astonishingly -- announces the person's chosen number.

Ta-dahhhhhh!!!!!!!!

Click here for the magic math cards! (This will open a new window. You can print the cards, then close the window to return right back here to find out how to perform the trick.)

Cut the 'cards' out and glue-stick them to card-stock, or at least heavy construction paper, so that they indeed become your reusable cards.

Now, the SECRET:

After making sure that the person for whom you are performing the trick has double-checked to make sure that whichever number he has selected is really on each and every one of the cards he has singled out -- and nowhere to be seen on the cards he pushes away, all you have to do is look at the the one number in the top-most right-hand corner of each card, and add THOSE numbers up in your head. If your friend has done a careful enough job separating the ones that have his number on them from the ones that do not, the little numbers in the top right corner of each card he has selected will ADD UP to his selected number!

Like magic!

-- Mitch