all possibilities;-
(G,B) = (0,4), (1,3), (2,2), (3,1), (4,0)
it is given that :-
atleast 1 out of 4 children is a girl:-
now new sample space would be:- (1,3), (2,2), (3,1), (4,0)
propability for exactly three girls:- means propability of (3,1):- i.e 1/4