Example #1a: Probability of a single event

Part a) demonstrates how to calculate the probability of a single event. If A is the event, then P(A) is the probability that A occurs.

The left side of the Applet below contains a circle with 20 marbles. The marbles are colored Red, Blue, or Gray and are numbered by either an Even or an Odd number. The experiment consists of randomly selecting a single marble. Assume the 20 possible outcomes are equally likely.

Since the outcomes are equally likely, the probability of an event is computed by dividing the number of outcomes in the event by the number of possible outcomes. For example, if the event is a Red marble, then:

P(Red)= # of Red marbles / # of marbles
=4 / 20.



Use the Applet to compute P(Blue), P(Gray), P(Even), and P(Odd). Observe which marbles are included in each event and verify the computed probabilities.

Part b): Conditioning

Part c): And Law

Part d): Or Law

Example #1