Question has a slight error .we should find n(A[tex]\cap[/tex]B)
[tex]\boxed{\sf n(A\cup B)=n(A)+n(B)-n(A\cap B)}[/tex]
[tex]\\ \rm\longmapsto n(A\cap B)=n(A)+n(B)-n(A\cup B)[/tex]
[tex]\\ \rm\longmapsto n(A\cap B)=12+9-16[/tex]
[tex]\\ \rm\longmapsto n(A\cap B)=5[/tex]
