Answer 1

We have: 

\[n\left( A \right) = 20, n\left( B \right) = 25 \text{ and } n\left( A \cup B \right) = 40\] 

We know: 

\[n\left( A \cup B \right) = n\left( A \right) + n\left( B \right) - n\left( A \cap B \right)\]
\[ \Rightarrow n\left( A \cap B \right) = n\left( A \right) + n\left( B \right) - n\left( A \cup B \right)\] 

 = 20 + 25  

\[-\] 40
 = 5