There are two ways to calculate probability - either by using math to predict, or by actually observing the event and keeping score.
Theoretical probability uses math to predict the outcomes. Just divide the favorable outcomes by the possible outcomes.
Experimental probability is based on observing a trial or experiment, counting the favorable outcomes, and dividing it by the total number of times the trial was performed.
Let's look at this example: we tossed a coin 36 times and recorded the outcomes:
H, T, H, H, T, T, H, T, T, T, T, T,
T, T, T, H, H, T, H, H, T, H, H, H,
H, T, H, H, T, H, H, T, H, H, T, H
Based on this experiment: