November 2017 Fiendish Function

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

  • GUESS
  • REPLY
  • 1
  • 9
  • 2
  • 16
  • 3
  • 21
  • 4
  • 24
  • 5
  • 25
  • 10
  • 0

The rule was: "Minus one times the number squared plus ten times the number".

Suppose the guess was 12:

  • Take -1 times the number squared, giving -144
  • Add 10 times the number, giving -24

So the result is -24.