1) Let the Probability of randomly meeting a four child family with exactly one child: P(A)
Let the Probability of randomly meeting a four child family with exactly 2 boy children : P(B)
Since the question is about how do we get to the Probability of meeting A or B
We can write:
P(A ∪ B) = P(A) + P(B) - P(A * B)
2) Knowing the subspace. We subtract to not count twice the Probability of A , and B.
If the events are mutually exclusives, i.e. there are no common elements so so we can write that
P(A ∪ B)= P(A) +P(B)