posted by Jope .
Suppose an event has a probability of p. What can you say about the value of p? What is the probability that the event will not occur? Explain
We can say that the value of p is bounded above by 1 and below by 0. An event can not have greater than a 100% chance of occurring, nor can it have less than a 0% chance of occurring, hence its boundary conditions of [0,1].
Given this, there are two possible outcomes. Occurring or not occurring. Since we know the probability of the event occurring is p, which is bounded above by 1; the probability of the event not occurring must be (1-p).
Alternatively, for lack of a better way of putting it, p can not occur and not occur at the same time. So the probabilities of p occurring and not occurring must add up to one.
Good luck, hope that helps :)