And what that means is if this event took place, this event cannot possibly take place. And likewise, if this event took place, this event could not have possibly taken place. And so because they're mutually exclusive, we apply the sum rule and we have one fourth plus one fourth. And that gives us one half. So one four plus one four gives us two eight or two four, which is equal to one half or 50%. So this is the probability of basically getting one girl on one boy.