Given a string, turn each character into its ASCII character code and join them together to create a number - let's call this number
total1:
'ABC' --> 'A' = 65, 'B' = 66, 'C' = 67 --> 656667
Then replace any incidence of the number 7 with the number 1, and call this number 'total2':
total1 = 656667
^
total2 = 656661
^
Then return the difference between the sum of the digits in total1 and total2:
(6 + 5 + 6 + 6 + 6 + 7)
- (6 + 5 + 6 + 6 + 6 + 1)
-------------------------
6