There are 20 students in a chemistry class and 30 students in a physics class. find the number of students which are either in physics or in chemistry class in the following cases:

a)two classes meet at the same hour. (hint: n(C∩P) =Φ)

b)The two classes meet at different hours and 10 students are enrolled in both the courses. (hint: n(C∩P)=10)

We have given, *n* (C) = 20, *n* (P) = 30 and *n* (C ∪ P) = ?

(i) When two classes meet at the same hour i.e. no student can attained both the classes.

∴ *n* (C ∩ P) = 0

We know that,

*n* (C ∪ P) = *n* (C) + *n* (P) – *n* (C ∩ P) = 20 + 30 – 0 = 50

(ii) When two classes meet at different hours and it is given that *n* (C ∩ P) = 10

∴ *n* (C ∪ P) = *n* (C) + *n* (P) – *n* (C ∩ P) = 20 + 30 – 10 = 40

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