Example 1 If P(A) = 7
13 , P(B) = 9
13  and P(A ∩ B) = 4
13 , evaluate P(A|B) Solution We have 
4
P(A
B)
4
13
P(A|B)=
9
P(B)
9
13
∩
=
=
Example 2 A family has two children What is the probability that both the children are
boys given that at least one of them is a boy © NCERT
not to be republished
PROBABILITY      535
Solution Let b stand for boy and g for girl