Intermediate Algebra (6th Edition)

We found that the probability of an event happening with certainty is 1. In other words, $P(x)=1$ tells us that there is a 100% chance that event x will happen, because the number of ways that x can occur is equal to the number of possible outcomes. Therefore, we can conclude that the probability of an event that is impossible to occur is 0, because there are 0 ways that an impossible event can occur. So the numerator of the probability formula will be 0 in this case.