If a and b are mutually exclusive events with p (a) = 0.70, then p (b) :

The overall probability can only be equal to 1. Since P (A) is already 0.70 and events A and B are mutually exclusive events, therefore:

P (B) cannot be larger than 0.30

This is so that the total probabilities would only be equal to 1:

P (A) + P (B) = 0.70 + 0.30 = 1