Question

A and B are two sets such that $A\cup B$ has $18$ elements If A has $8$ elements and B has $15$ elements then the number of elements in $A\cap B$ will be:

• A. $5$
• B. $8$
• C. $7$
• D. $4$

Answer 1

The correct option is A $5$

$n(A\cup B)=n\left(A\right)+n\left(B\right)-n(A\cap B)$

$n(A\cap B)=n\left(A\right)+n\left(B\right)-n(A\cup B)=8+15-18=5$