A person is trapped inside a room, unable to be free unless he gives the correct answer to a riddle. He observes the two people ahead of him give the answers as follows:
the first prisoner asks to be let out. The spirit guarding the door hisses "twelve" and the prisoner answers "six". He is let out through the door.
the second prisoner asks to be let out. The spirit guarding the door snarls "six" and the prisoner answers "three". He is also let out.
The third prisoner, having decided he knows the logic, asks to be freed. The spirit guarding the door growls "ten", and the prisoner answers "five". He was NOT allowed out.
why?