If P(not event B) = 0.95, what is P(event B)?

The two events of B happening, or B not happening, are complementary events. One or the other must happen. There is no overlap so both events cannot happen simultaneously. This means the two probabilities add to 1, which represents 100%

P(event B) + P(not event B) = 1

P(event B) = 1 - P(not event B)

P(event B) = 1 - 0.95

P(event B) = 0.05

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