The Brainliest Answer!
Let set A=no of students who opted physics (⇒n(A)=55) and
Let set B=no of students who opted maths.(⇒ n(B)=65).
Then ,
A U B=Total no of students.
Therefore , n(A U B)=100
Therefore , no.of students studying only physics=n(A-B)
Formula is:
n(A-B)= n(A U B) - n(B)
Or, n(A-B)=100-65
Therefore , 35 students opted only physics (ANSWER)

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OH I SEE.It looks like you put up a meaningful/unmeaningful answer somewhere and it was reported/deleted by anjali2000.You have a somewhat _______ against her (FILL IN THE BLANKS)
she has not answered any of my questions till now.she deletes the answers which are correct.we are the beginners,the moderates should guide and tell us about our mistakes.if they know the correct answer,then why don't they give the perfect answers to the deleted questions???tell me,am i wrong or is she wrong in deleting almost everyone's answer simply without any reason??????
How do u know that she deletes answers which are correct?Do u get them verified from others?If not,then i would request you not to continue discussion about this topic as then it will lead to further bitterness and enemity.
    n(A) =no.of students took physics = 55
    n(B) =no.of students took maths = 65
    n(A U B) = no.of students in the school = 100
    n (A П B) = no of students took both = n(A) + n(B) - n(A U B)
                                                         = 65 + 55 - 100
                                                         = 20
number students took only physics = n(A) - n (A П B)
                                                    = 55 - 20
                                                    = 35 students took only physics