The sum of the probabilities of all of the possible events should be equal to 1, and that makes sense because you're adding up all of these fractions, and the numerator will then add up to all of the possible events. The denominator is always all of the possible events, so you have all of the possible events over all of the possible events when you add all of these things up. Now let's take it up a notch. Let's figure out the probability of... I'm going to take this coin, and I'm going to flip it twice. The probability of getting heads and then getting another heads. The probability of getting a head and then another head.