In a survey of 200 students of a school, it was found that 120 students study Mathematics 90, study Physics, and 70 study Chemistry, 40 study Mathematics and Physics, 30 study Physics and Chemistry, 50 study Chemistry and Mathematics and 20 none of these subjects. Find the number of students who study all three subjects.

Formula:- n (A∪B∪C)’ = n(U) – n (A∪B∪C)

Let A be Mathematics i.e. n(A) = 120

B be Physics 120 i.e. n(B) = 90

and C be Chemistry i.e. n (C) = 70

According to question

n(A∩B) = 40, n(B∩C) = 30, n(C∩A) = 50, n(A∪B∪C) ’ = 20

n (A∪B∪C)’ = n(U) – n (A∪B∪C)

⇒20 = 200 – n (A∪B∪C)

⇒n (A∪B∪C) = 200 – 20 = 180

⇒n (A∪B∪C)= n(A) + n(B) + n(C) – n( A∩B) – n( B∩C) – n( C∩A)+ n(A∩B∩C)

⇒180 = 120 + 90 + 70 - 40 - 30 - 50 + n(A∩B∩C)

⇒ n(A∩B∩C) =20