among the 16 cities that a professional society is consedering for its next 3 annual convention, 7 are in the western pa

[answerhappygod]

among the 16 cities that a professional society is consedering
for its next 3 annual convention, 7 are in the western part of the
united state. To avoid arguments, the selection is left to chance.
if none of the cities can be choose more thamn once, what are the
probabilities that (a) None of the conventions will be held in the
western part of the united state. (b) All of the conventions will
be held in the western part of the united states?