Let T = set of students who take tea
C = set of students who take coffee
M = set of students who take milk
∴ n(T) = 25, n(C)
= 20, n(M)
= 15,
n(T ∩ C) = 10,
n(M ∩ C) = 8,
n(T ∩ M) = 0,
n(T ∩ M ∩ C) = 0
∴ Total number of students in the hostel = n(T ∪ C ∪ M)
= n(T) + n(C) + n(M) – n(T ∩ C) – n(M ∩ C) – n(T ∩ M) + n(T ∩ M ∩ C)
= 25 + 20 + 15 – 10 – 8 – 0 + 0
= 42