**Theoretical
Method:** Make mathematical hypotheses to determine probability of an event.

The four properties of probability:

1. Event occurs.

P (x occurs) + P (x does not occur) =
1

2. Occurrence at different times; when
x and y occur at different times.

P (either x or y occurs) = P(x occurs)
+ P(y occurs)

3. X and y are independent. Y is not
more likely to happen when A happens than when A does not happen.

P(x and y both occur) = P (x occurs)
* P (y occurs)

4. Other influxes. When x occurs, y has
to occur, too.

P (x occurs) <= P (y occurs)