A family has three children. What is the probability that at least one of them is a boy? According to me there are $4$ possible outcomes:
$$GGG \ \ 
BBB \ \ 
BGG \ \
BBG $$
Out of these four outcomes, $3$ are favorable. So the probability should be $\frac{3}{4}$.
But should you take into account the order of their birth? Because in that case it would be $\frac{7}{8}$!

The possibilities are
ggg ggb gbg bgg gbb bgb bbg bbb 
at least one boy... 7/8
