Multiply your phone number, a seven-digit number
(no area code) by the number 8.
Then write down the following three numbers:
(i) your phone number,
(ii) the number 8,
(iii) the product of your phone number times 8.
Add up the numeric values all the individual digits in those three numbers.
If the sum is more than one digit, take that sum and add up its digits.
Continue adding up digits until only one digit is left.
What is that single final digit?
It will always be the same number. Why ?
Created by Martin Gardner.
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