Weigh 3 balls, then weigh 3 more. If the 2 weightings are identical then the lighter ball has not yet been weighed, so replace one ball until you find it. If one of the weightings is lighter, go back to that configuration and remove one ball at a time until you see an inconsistent drop in weight.
(Chose Python) class Class(object): def __init__(self): pass def getter(self): pass def setter(self): pass ... etc. Interviewer had specific questions about the class or what properties need to be added