1. Get a calculator. (You won't be able to do this one in your  head)

2. Key in the first three digits of your phone number (NOT the area code)

3. Multiply by 80

4. Add 1

5. Multiply by 250

6. Add the last 4 digits of your phone number

7. Add the last 4 digits of your phone number again.

8. Subtract 250

9. Divide number by 2

Do you recognize the answer? 
















 








Explanation:

Let x= 1st three digits of phone number
let y= last 4 digits of phone number

x
80x
80x + 1
250(80x + 1) = 20000x + 250
20000x + 250 + y
20000x + 250 + 2y
20000x + 2y
10000x + y

This last step shows why you get your phone number back.  
x (which represents your first three digits of your phone number)
   is multiplied by 10000, which just moves it over 4 places and 
   then y is put in the four places on the right.