HS #123 2025.10.9
Mathematics and other Magic
Here’s a riddle my father told me when I was young: Three men enter a hotel and pay $10 apiece for their rooms – a total of $30. After they go to their rooms, the manager realizes he has overcharged them, so he gives the bellhop $5 to split between them. But the dishonest bellhop pockets $2 and then gives each of them $1.
So each of the three men has now paid $9. That’s $9 x 3 = $27. Plus there is the $2 which the bellhop kept. $27 + $2 = $29. Where did the extra dollar go?
It’s simple 3rd grade arithmetic – try to solve it before reading further.
Nope – only 30% of you have tried to solved it. We don’t go on until everyone tries.
OK – some of you have now succeeded. I’ll explain it to the rest of you. First, where are the 30 dollars? The manager has $25, the bellhop has $2, and each of the three men has $1 - a total of 30.
So how did I intentionally confuse you with my question? I misdirected your attention by suggesting that you ADD $27 and $2 together, even though there was no reason to do so. Instead of adding them, you should have subtracted the $2 from $27. Why? Because subtracting them answers the question: How much money did the hotel make? The hotel made 3 x $9 = $27 MINUS the $2 which the bellhop kept which gives: $27 - $2 = $25.
This sort of intentional misdirection lies behind much magic.
Years ago, Barbara Walters had the psychic Uri Geller on her show. As Walters and viewers watched, Geller seemingly effortlessly bent a thick key just by lightly stroking it with his finger. As I understand, Walters called it one of the most spiritual moments of her life. Years later, the Amazing Randi reproduced the trick for Walters, showing how both he and Geller had quickly bent the key when he had diverted Barbara’s attention. Then, with the key already bent, he held it in such a way as to conceal that it was bent, and then made it look as if he were effortlessly bending it with (he claimed) psychic power. I have fun entertaining my math students with similar tricks.
Now back to mathematics. Have you heard of Zeno’s Paradoxes? Zeno was a math-magician of the highest caliber who has misdirected the thoughts of his readers for millennia - usually involving the infinite. Here’s an example: A hare and a tortoise are in a race in which the tortoise has a 10-meter head start, and the hare runs ten times faster than the tortoise. Zeno argued that it’s impossible for the hare to pass the tortoise because by the time the hare has run to the starting point of the tortoise, the tortoise has run another meter. By the time the hare reaches that point, the tortoise has run 1/10th of a meter. By the time the hare has reached it, the tortoise in turn has run 1/100th of a meter and so on. Thus, whenever the hare has reached where the tortoise was, the tortoise has advanced. Conclusion: The hare can never pass the tortoise.
Isn’t this mind-boggling? Of course, we know it’s not true - there must be a flaw in the reasoning somewhere because obviously the hare will in fact pass the tortoise. But where’s the flaw? If we didn’t know the truth from experience, we’d likely find the argument convincing.
As with the other examples, your gaze has been misdirected. If you took a pie and split it in half, then again, then again, and continuing, you would get increasingly smaller pieces. But putting them all back together again you would still get just the one pie. Agreed? Cutting the pie this way illustrates that ½ + ¼ + 1/8 + 1/16 + 1/32 + . . . = 1. Conclusion: An infinite series of numbers can sum to a finite number.
The hare and tortoise example is similar. The number of seconds needed to catch the tortoise (assuming the hare runs 1 meter/sec) is 1 + 1/10 + 1/100 + . . . = 1.1111 . . . which is 10/9 seconds (check with your calculator). Zeno tried to confuse you – to misdirect your gaze – by breaking that time into an infinite number of pieces. Now that you know his trick, you can sleep again at night.