(a)
Here, $P(B) = 1 - P\left( B^{'} \right) = 1 - \frac{2}{5} = \frac{3}{5}$
$\therefore\ P(A \cup B) = P(A) + P(B) - P(A \cap B)$
$= \frac{4}{5} + \frac{3}{5} - \frac{1}{2} = \frac{9}{10}$
$\therefore\ P\left( A \cap B^{'} \right) = P(A \cup B) - P(B)$
$= \frac{9}{10} - \frac{3}{5} = \frac{3}{10}$