my bad i got it... the concept behind the question is.. probability of an event can never be -ve
so first when we take the limits of each event's probability we get [-1/3,1/2]
and since they are mutually exclusive..since... 0<p(AUBUC)<1 ... 0</= p(a)+p(b)+(c) </= 1
sry abt the sign i didnt know how to represent lesser than or equal to in here..
you will get the ans when you do the 2nd step