Calculator Tricks 4
By
Brian Carruthers
(Please see notes on slide 1)
Lucky 7
or
Unlucky 13
Step One
Think of a three-digit number
 Enter it twice on your
calculator.
 For example: 123123
 Now concentrate on the
display.

Step Two
I think that your number is
divisible by 11.
 Do you believe me?
 If you don’t then I’ll prove
it to you.

Use your calculator to check
that your number is divisible
by 11.
Am I right?
Step Three
But, guess what:
 Your answer displayed on your
calculator is divisible by 13.
 Again, use your calculator to
check the answer.

Now, divide your result by
your original three-digit
number.
 Your final answer will be :

7
Watch this:




Enter your original three-digit
number twice on your calculator.
Divide that number by 11.
Now divide your result by 7.
Divide your answer by your
original three digit number.
The answer will be
13
Why does this work?




Entering a three-digit number twice
For example: 123123 is the same
as multiplying it by 1001.
123 x 1001 = 123,123.
Because 1001 = 7 x 11 x 13, the
six-digit number will be divisible by
7, 11, 13, and your original threedigit number.
Watch this:
345345 divided by 11 = 31395
31395 divided by 13 = 2415
2415 divided by 345 = 7
678678 divided by 11 = 61698
61698 divided by 7 = 8814
8814 divided by 678 = 13.
Now what did you think
of that?