The probability of an event that is certain to occur is always 1, or 100%. In probability theory, probabilities range from 0 to 1, where 0 represents an impossible event, and 1 signifies an event that will definitely happen.
To understand why certain events have a probability of 1, consider an example: the event of the sun rising tomorrow. While unforeseen circumstances could theoretically change this outcome, practically speaking, the sun rising is seen as an event that will happen without fail. Thus, we can assign it a probability of 1.
Another perspective is to think about a probability space, which is a mathematical construct of all possible outcomes of an experiment. An event with probability 1 is said to occur in all scenarios of this space. Conversely, if an event has a probability less than 1, that means there are possible outcomes where the event could fail to occur.
In summary, when we talk about events in the realm of probability, an event that is certain to occur definitively carries a probability of 1.