Question

Let $A=\{1,3,5,7,9,11\},$ $B=\{3,9,27,81,243\},$ then $n\left[\right(A\times B)\cap (B\times A\left)\right]$ is

Answer 1

$(A\times B)\cap (B\times A)=(A\cap B)\times (B\cap A)$
$\therefore A\cap B=\{3,9\}\Rightarrow 2$ elements
$n\left[\right(A\times B)\cap (B\times A\left)\right]=2\times 2=4$