Originally Posted by

**Plato** These two quotes prompt me to ask if either of you have had any training in formal logic?

The two logical connectives, *and* & **or** serve similar functions while being totally at odds.

$P\text{ and }Q$ means that both P & Q must be true; while $P\text{ or }Q$ means that at least one of them is true.

So if a boy has an older brother **and** a younger sister then that is only one case $\bf{BBG}$ There can be no other.

So if the question were: In a family of three children, what is the probability that a boy has an older brother **and** a younger sister? Then the answer is $\dfrac{1}{8}$.

Now if the question were: In a family of three children it is known that at least one is a boy, what is the probability that boy has an older brother **and** a younger sister?