Find the probability that when a couple has three children, at least one of them is a girl. (assume that boys and girls are equally likely.)

Consider this option (2 ways for this):
1. Probability=1-[probability_all_boys].
for [probability_all_boys]=1/[all_possible_combinations];
for [all_combinations]=2³=8. It means that 
Probability=1-(1/8)=7/8=0.875.
2. Probability=[needed_cases]\[all_possible_cases];
all possible combinations are:
bbb;ggg;bgg;bgb;bbg;gbg;gbb;ggb; - (where 'g' - girl, 'b' - boy) total 8.
needed cases: ggg;bgg;bgb;bbg;gbg;gbb;ggb, - total 7.
Probability=7/8=0.875