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.
