March 2017 Fiendish Function

The Fiendish Function posed during March 2017 had the following sample guesses and replies:

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

The rule was: "Count the number of letters in the word for the number, then count the number of letters in the word for that number".

Suppose the guess was 21:

• Count the number of letters in "twenty-one", giving 9.
• Count the number of letters in "nine", giving 4.

So the result is 4.