By definition of conditional probability,
P(B | A) = P(A ∩ B) / P(A)
==> P(A) = P(A ∩ B) / P(B | A) = 0.15/0.75 = 0.2
By definition of complement,
P(B') = 1 - P(B)
==> P(B) = 1 - P(B') = 1 - 0.7 = 0.3
Now by the inclusion/exlcusion principle, we have
P(A U B) = P(A) + P(B) - P(A ∩ B)
==> P(A U B) = 0.2 + 0.3 - 0.15 = 0.35
