Final answer:
The probability of the union between mutually exclusive events A and B is equal to the sum of their individual probabilities.
Step-by-step explanation:
When two events, A and B, are mutually exclusive, it means that they cannot happen at the same time. So, the probability of both A and B occurring together is 0.
In this case, if the probabilities of A and B happening individually are given as P(A) = 0.3 and P(B) = 0.4, respectively, then the probability of either A or B happening (the union of A and B) can be calculated by adding their individual probabilities:
P(A OR B) = P(A) + P(B).
Therefore, the probability of the union between A and B is 0.3 + 0.4 = 0.7.