(b) Here, $P\left( B^{'} \right) = 1 - 0.4 = 0.6$ and $P(A) = 1 - 0.3 = 0.7$ $\therefore\ P\left( A \cup B^{'} \right) = P(A) + P\left( B^{'} \right) - P(A \cap B^{'})$ $= 0.7 + 0.6 - 0.5 = 0.8$
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