[tex]p(a \: \cup \: b) = p(a) + p(b) - p(a \: \cap \: b) \\ p(a \: \cup \: b) = 0.2 + 0.4 - p(a \: \cap \: b) \\ p(a \: \cup \: b) = 0.6 - p(a \: \cap \: b) \\[/tex]
[tex]but \: \: p(a \: \cap \: b) = p(a /b) \times p(b) \\ p(a \: \cap \: b) = 0.25 \times 0.4 = 0.1[/tex]
[tex]p(a \: \cup \: b) = 0.6 - p(a \: \cap \: b) \\ p(a \: \cup \: b) = 0.6 - 0.1 \\p(a \: \cup \: b) = 0.5[/tex]
