March 2015 Fiendish Function

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

• GUESS
• REPLY
• 1
• 1
• 2
• 2
• 3
• 3
• 4
• 3
• 5
• 4
• 10
• 7

The rule was: "Write the number in binary, write it in binary, then add the digits together".

Suppose the guess was 26:

• Write the number in binary, giving 11010.
• Write the number in binary, giving 10101100000010.
• Find the number's digits, giving 1, 0, 1, 0, 1, 1, 0, 0, 0, 0, 0, 0, 1, 0
• Add these together, giving 5.

So the result is 5.