Gate guardian

The Gate Guardian is a riddle]. It can be played everywhere.

Equipment

 * You don't need any equipment.

Procedure
The trainer tells the following story:
 * The gate of an English town is guarded by a guardian. Only people that can prove their citizenship in that town may enter. The guardian asks a riddle to everybody who wants to enter. Only those people that know the riddle and can thus give the correct answer may enter. You don't know the answer, but you can listen to a few questions and their correct answers. Can you find the correct answer?
 * A citizen approaches, the guard asks "28". The citizen gives the correct answer "11".
 * The next citizen is asked "11". He correctly answers "6".
 * The following citizen is asked "6", and correctly answers "3".
 * When asked "3", the correct answer would be "5".

Can you find the system, by listening to those and probably a few more examples?

Solution
While finding the correct answer by actively playing and guessing, you can find it on the page gate guardian/solution.

Torwächter