n(S) = 20 + 60 = 80
Let A: student is a boy n(A) 60
P(A) =
60
80
60
=
3
4
3
Let B: student is a class holder
1 class = 30 boys + lo girls = 40
P(B) =
40
80
40
=
1
2
1
Let (A ∩ B) = n(Student is a boy and I class holder) = 30
P(A) = (A ∩ B) =
30
80
30
=
3
8
3
∴ P(Student is a boy of first class holder)