Question

Let A and B be two sets such that : $n\left(A\right)=20$, $n\left(A\cup B\right)=42$ and $n\left(A\cap B\right)=4$. Find n(B)

Answer 1

$n\left(A\right)=20$

$n(A\cup B)=42$

$n(A\cap B)=4$

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

$42=20+n\left(B\right)-4$

$n\left(B\right)=26$