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)