An event must either occur ( E ) or not occur ( ! E ) Therefore the sum of the probabilities of an event occurring and an event not occurring must be equal to 100% That is P ( E ) + P ( ! E ) = 1.00 Given that P ( ! E ) = 0.40
This implies that XXX P ( E ) + 0.40 = 1.00 XXX P ( E ) = 0.60