There is a land where every inhabitant is either a day-knight or a night-knight. Day-knights tell the truth in the day and lie at night. Night-knights lie in the day and tell the truth at night.

Members of the Smith family are day-knights and members of the Jones family are night-knights.

Three inhabitants Al, Ben and Cal approach a visitor to the island. It is known that precisely two of them are members of the Jones family and the remaining one is a member of the Smith family.

They say:

Al: Ben belongs to the Jones family. 
Ben: Cal belongs to the Jones family. 
Cal: Al’s surname is Smith.

What are the surnames of the three inhabitants? Is it day or night?

which is Smith   D/N     stmt that can't exist
                         (not necessarily the only one)
    A             D       Ben's
    B             D
    C             D       Cal's
    A             N       Al's
    B             N       Al's
    C             N       Ben's

   
So it's Ben Smith telling the truth; the others are Joneses and it's daytime.
   

Posted by Charlie on 2013-11-25 14:01:08