(b)
Given, $A \cup B = S$
$\therefore P(A \cup B) = P(S) = 1$
$\Rightarrow P(A) + P(B) = 1\ \ \ \ \ \lbrack\because P(A \cap B) = 0\rbrack$
$\Rightarrow P(A) + 2P(A) = 1$
$\Rightarrow P(A) = \frac{1}{3}\ \ \ \ \lbrack\because P(B) = 2P(A),\ given\rbrack$