Answer
if $B\subseteq A$
Work Step by Step
CardinaIity of a Union:
If A and $B$ are finite sets, then
$n(A\cup B)=n(A)+n(B)-n(A\cap B)$.
So the equality $n(A\cup B)=n(A)$ stands if
$n(B)-n(A\cap B)$=0, that is,
$n(B)$=$n(A\cap B)$.
This can only be if B is contained in A, or equal to A.
