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).
A U B=Total no of students.
Therefore , n(A U B)=100
Therefore , no.of students studying only physics=n(A-B)
n(A-B)= n(A U B) - n(B)
Therefore , 35 students opted only physics (ANSWER)