Tuesday, January 01, 2008

Mathematical Mind Reading Revisited

Here’s another mathemagic trick you can learn that will astound your friends, neighbors, children, or parents (referred to as your “victims,” below) – and perhaps instill in them a new-found respect for and interest in the entertainment value of basic algebra.

For example:

"Pick any number, and you must remember this number (the victim thinks whatever, maybe 2, or 7 – but you think "x").

Multiply by 4. (Victim thinks: 8 or 28, you think 4x.)

Add 5. (Victim: 13 or 33, you: 4x + 5.)

Add 6. (Victim: 19 or 39, you: 4x + 11.)

Subtract 3. (Victim: 16 or 36, you: 4x + 8; notice both parts of “4x + 8” divide evenly by 4.)

Divide by 4. (Victim: 4 or 9, you: 1x + 2 or simply x + 2; notice that you've now returned to "x," the original number.)

Now ... SUBTRACT YOUR ORIGINAL NUMBER. (The “x” is taken away! victim: 2 or 2, you: 2.)

You now know exactly what your victim is thinking! So, now, you can add, subtract, multiply, and divide to your heart's content, knowing with absolute certainty that you are accurately “reading” your victim’s mind. Finally, at the end of your list of instructions, announce with great fanfare (rubbing your temples, etc.) what number the victim is thinking of ... and notice with wry smile the dropped jaw and blank stare of amazement.

Note: to avoid mistakes that could spoil your performance, first hand your victim a calculator to use, and be sure that your first two instructions are to multiply and/or add; this way you'll avoid the possibility of your victims having to struggle with troublesome negative numbers.

This trick is especially effective with a group of victims (say, an entire classroom of fellow students, or the guests at a dinner party) all following your instructions simultaneously. No matter what numbers the various victims think of initially, as soon as you give the "subtract the original number" instruction, the variable is eliminated, and you are ALL now thinking of the same number, no matter what. VERY impressive! That's the beauty of algebra: that you can work with unknown numbers just as you do with known numbers, because numbers are numbers, whether known or unknown, and always obey the same rules.

With practice, entirely new routines similar to the example above can be improvised on the spot, at will, several times in a row if necessary, to convince your victim of your uncanny psychic powers. You can then, if you wish, show your victims (preferably with pencil and paper handy) how easy it is to perform the trick using basic algebra.


• First practice this trick by yourself several times, using paper and pencil, playing the roles of victim and mind reader, until you're very confident that you can easily and correctly perform it.

• It’s a good idea to give your victim a calculator to work with (so they don’t make mistakes – which will make you look bad).

• Make your second instruction an addition command, to avoid having to work with negative numbers.

• It's helpful to obfuscate the trick involved by instructing victims to "add the numbers of fingers in your left hand" or "divide by the number of A's in America" instead of merely saying "add 5" or "divide by 2."

Have fun!


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