A couple has $3$ children.
1) What is the probability that three are girls if the eldest is a girl?
2) Solve the same problem, provided that it is known that one of the children is a girl.
1) Let $A$ be the event that the $3$ children are girls. And let $B$ be the event that the eldest is a girl. The sample space is $GGG$ $BGG$ $BBG$ $BBB$.
Since it is given that the eldest is a girl, then $P(B)=\dfrac34$.
$P(A$ and $B)=\dfrac14$
2) I am confused about the second part. Give me a hint, please.