Sunday, June 10, 2012

mind reader or math trick?

mind reader or math trick?

Here is a math trick that will make you appear to have a mental ability to read the minds of others.  This is one of those math tricks that will really impress your audience, no matter the size of your audience.

First, have your friend secretly choose a three-digit number that is not a repeat of the same digit (e.g., not 222 or 333)

Your friend writes down “895”

Now, have your friend rearrange the numbers without revealing the result

Your friend writes down “589”

Next, have your friend determine the difference between the numbers, still keeping the result secret

Your friend writes down “895 – 589 = 306”

Now have your friend remove one of the digits of result, without telling you which one

Your friend scratches out the zero, “306”

Now ask your friend what the remaining digits are

Your friend says, “3 and 6”

Ask your friend to think very hard of the number that was removed.  Close our eyes and make it appear that you are deeply concentrating, trying to pick up your friend’s thoughts.  Now, with confidence, you can tell your friend the number that was removed!

What’s that you say?  You can’t pick up your friend’s mental thoughts?  Well, if you don’t have this mental ability, there is another way of determining the number that was removed.

The differences of different numbers consisting of the same digits, when added together, form multiples of 9.  In our case above, the sum of the digits 3, 0, and 6 is 9.  So given the digits 3 and 6, it is easy to determine that the missing digit is 0!

Here is another example

Your friend has the remaining digits:

1, 3

What is the remaining digit?  It has to be 5, since 5 = 9 – 1 – 3.

In this example, your friend may have chosen


With a rearrangement of


The difference is


and the number 5 is deleted, resulting in the digits


This trick is not restricted to three-digit numbers.  You can use as many as you wish!

Here is an example using 5 digits:

Your friend chooses




With the difference being


Deleting the 8, the remaining digits are


Here it is easy to see that the missing digit must be an 8, since all the digits but the “1” are 9s.

Have fun with this math trick.  Most people will be stumped as to how you do it, so be sure NOT to tell them how it is done!

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