A Mathematical Proof for Prince Prasad’s Number Trick
9x =9 ({ [ ( x – 1 ) – { 9x – [ 10 ( x – 1 ) ] } + 5 ] / 2 } + 3)
In my younger days, I was often intrigued by magicians and their mysterious ability of making things appear and disappear but when I got older, it was the other things that made magic a success that truly intrigued me. Just like you, I tend to question the angle of perception directors don’t wish but force into the eyes of audiences and all the dry-ice smoke about a magician they want us to feel the spell we’re under.
Perhaps you’re a fan of David Blaine and you’re most intrigued by his mentalist feats, unfortunately, I am not here to discuss the sort of magic David Blaine performs. However, I am certain this simple “mind-reader” trick would work as well as David’s when you do it with K-6 math students.
I am not really into the magic within this trick but the magic is the formula that makes this trick work. I invented this process when I was 14 about 9 years ago and have reused the method in performing card tricks with additional constant component operations to obtain the suit of the cards. The effect of this trick is a divined number that happens to be the number chosen by the spectator
Method.
1) First find a willing and participating spectator.
2) Get him/her to choose and think of a single digit number between 0 – 9 without having to say it out loud.
3) Tell the spectator to multiply the number they chose by 9.
4) Tell them their product is a two digit number even if they chose the selected number was 1. Tell them why: because 1 x 9 = 09.
5) Now that they know they have a two digit product, tell them to subtract the digits of the product from left to right. For example: 2 x 9 = 18 =>1 – 8 = -7
6) Tell them that they should never throw away the negative sign.
7) Now tell them to multiply the result by 5.
8) Then get them to divide that product by 2.
9) Finally tell the spectator that if you were going to reverse the process to obtain the number they were thinking of it would take as long it took to get to the last step which is the quotient of the result of the process with a denominator of 2. With that get them to tell you the result in step 8.
10) Quickly add 3 to the number the spectator gives you and there you have it, the number they were thinking of.
Just one piece of advice don’t repeat this trick in the same place with the same person twice or you’ll get busted and that won’t be such a nice trick after all when it’s a K-6student you are trying to con.
Anyways, here’s what’s more interesting than the trick itself, the mathematical proof for the process.
Assuming the number the spectator is thinking of is denoted by the a single letter x, to represent the variable and the multiplier given is 9, then the process described above can be represented mathematically as below:
From 9x =9 ({ [ ( x – 1 ) – { 9x – [ 10 ( x – 1 ) ] } + 5 ] / 2 } + 3)
If 9 * x = 9x,
then x =9 ({ [ ( x – 1 ) – { 9x – [ 10 ( x – 1 ) ] } + 5 ] / 2 } + 3)/9.
Further expanding the function gives us;
x = [2x – (11 – 5)]/2 +3
= x – 3 + 3x = x, hence the function of the trick is proven.
Hope you enjoyed this and I would be glad if you could share some trick functions/proofs with me. Contact me anytime via e-mail or by clicking on the contact me link at the top. By the way the alter-moral of my story is: Don’t lie! You can keep adding to a lie but eventually you’ll blow the truth up out in the open. Magic is nothing without mathematics!