Math Movement

On October 21, 2010, over 60 sites around the world celebrated the life of Martin Gardner. Davidson College was among the locations. Our time together was filled with fun, math, and laughter – a fitting evening in honor of an influential man. While having no formal education in mathematics, Martin Gardner had an enormous influence on the subject, reflected in the words of mathematician Richard K. Guy that Gardner brought “more mathematics, to more millions, than anyone else.” While Martin Gardner created a plethora of playful mathematical enticements, let’s experience some math magic! This trick is taken from Gardner’s 1956 book Mathematics, Magic and Mystery (p. 59-60).

The trick begins after placing a group of pennies in the configuration of the number 9, as seen below.

Now simply follow the steps below:

1. Pick a double digit number less than 20. (The trick works with any double digit number, actually, but for now let’s pick one less than 20.)
2. Starting at the tail of the 9, count up and around the nine counter-clockwise until you reach your number. When you reach your number, click that last penny that you counted. (This should leave pennies that lie in a circle.)
3. Beginning at that last coin touched and counting it as 1, continue to count clockwise around the circle until you again reach your selected number. Keep this penny in mind as you again click the image of the 9.
4. Did the magic of Gardner predict your penny?

How did it work? Click through the images a few more times and see if you can uncover the mathematical magic! It’s magic so I won’t tell you the secret but the magic is quite a tail! (Pun is entirely intended.)