Assume that phone number is "xyz"-"abcd" Then, the steps become: 2) First 3 digits: xyz 3) Multiply by 80: xyz times 80 4) Add 1: xyz times 80 + 1 5) Multiply by 250: xyz times 20,000 + 250 6) Add the last 4 digits: xyz times 20,000 + 250 + abcd 7) Add the last 4 again: xyz times 20,000 + 250 + abcd times 2 8) Subtract 250: xyz times 20,000 + abcd times 2 9) Divide by 2: xyz times 10,000 + abcd which equals => x,yz0,000 + abcd which equals => x,yza,bcd (i.e. your phone number) Will work for any 7 digit number. Any number of other math sequences could be done to provide the same result.
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