From the set of all families having three children, a family is picked at random
If the eldest child happens to be a girl, find the probability that she has two brothers.

Explanation

A family is picked up at ramdom from a set of families having 3 children.
The eldest child happens to be a girl. We have to find the probability that she has two brother. Let G denotes a girl and B denotes a boy.
`therefore P (B)=1/2,P(G)=1/2`
P (BB | G ) =P(B) xx P (G)=1/2xx1/2=1/4`