August 2018 Fiendish Function

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

  • GUESS
  • REPLY
  • 1
  • 3
  • 2
  • 10
  • 3
  • 15
  • 4
  • 36
  • 5
  • 45
  • 10
  • 170

The rule was: "Write the number in binary, write it twice, then interpret it as binary".

Suppose the guess was 26:

  • Write the number in binary, giving 11010.
  • Write it twice, giving 1101011010.
  • Interpret the number as binary, giving 858.

So the result is 858.