October 2015 Fiendish Function

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

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

The rule was: "Write the number in binary, reverse it, and then convert it back to decimal".

Suppose the guess was 19:

• Write this in binary, giving 10011.
• Reverse it, giving 11001
• Convert this back to decimal, giving 25.

So the result is 25.