could be a standard formula you have presented.this standard formula is applicable to all telephone directory systems?[/DIV]
just playing with numbers really........
multiplying by 80, then 250.......is multiplying 20,000...the 4 zeros provide room for the last 4 digits of the phone number....this large number is divided by two which in effect the original 3 digits were multiplied by 10,000
adding 1....multiplying by 250 = 250....which is deducted off in step 8
the last 4 digits are added twice and divided by 2 = last 4 digits
am I making sense?