September 2018 Fiendish Function

The Fiendish Function posed during September 2018 had the following sample guesses and replies:

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

The rule was: "Count the number of characters in the word for the number, then add one".

Suppose the guess was 21:

  • Count the number of characters in "twenty-one", giving 10.
  • Add one, giving 11.

So the result is 11.