(A) For independent events:
\[ P(A \cap B) = P(A) \cdot P(B) = 0.5 \cdot 0.4 = 0.2. \](B) Using the formula:
\[ P(A \cup B) = P(A) + P(B) - P(A \cap B). \]Substituting values:
\[ P(A \cup B) = 0.5 + 0.4 - 0.2 = 0.7. \]There are two children in a family. If it is known that at least one child is a boy, find the that both children are boys.