>>11337997You would need to look at two things: the sample space (all things of the type happening) and the number of occurences (what you want to have happen).
In your tennis example, since there are two people playing tennis usually (sample space) you want one of them to win. That is a probability of 1/2. Of course, the winner of tennis match is "resolved" at random which is why it is a 50% "chance". Do check up other instances of probability at work with this basic idea in mind.
