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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