Question

Let $A$ and $B$ be two finite sets such that $n(A-B)=30,n(A\cup B)=180$. Find $n\left(B\right)$.

Answer 1

$A$ and $B$ are finite sets

$n(A-B)=30$

Elements in set A that does not contain elements of set $B$.

$n(A\cup B)=180$

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

$=180-30$

$=150$

$n\left(B\right)=150$