September 2015 Fiendish Function

The Fiendish Function posed during September 2015 had the following sample guesses and replies:

  • GUESS
  • REPLY
  • 1
  • 1
  • 2
  • 3
  • 3
  • 6
  • 4
  • 1
  • 5
  • 6
  • 10
  • 7

The rule was: "Add my previous answer to the number, then add the digits together".

Suppose the guess was 13:

  • Add my previous answer to the number.

For example, if my previous answer was 10 this gives 23.

  • Find the number's digits, giving 2, 3
  • Add these together, giving 5.

So the result is 5.