#### Answer

$550$

#### Work Step by Step

Here $A$ is the set attending to the session I and $B$ is the set attending to the session II.
According to the counting formula we know that $n(A\cup B)=n(A)+n(B)-n(A\cap B).$
Hence here $n(A\cup B)=n(A)+n(B)-n(A\cap B)=15+20-10=25.$
Hence: $n(A\cup B)=200+150-75=275.$
We know that $275$ students attend neither of the lectures, hence in total there are $275+275=550$ students participated.