Question

From the given diagram, find number of elements in B${}^{{}^{\prime }}$

Answer 1

$B^{\prime }$ will have all the elements outside the oval drawn for $B$.
From the figure, we have
$B^{\prime }=\{3,4,6,7,8,9,10\}$

Hence, $n\left(B^{\prime }\right)=7$.