This applet can be used to/explore the addition law for mutually exclusive events.
e.g. Using the default tank, highlight region A and ask what is the probability of fish being in region A, P(A)?
By considering the areas
Uncheck and check D. Whats is the probability of a fish being in region D, P(D)?
Check A... what is the probability of the fish being in region A or D. Now it should be evident that this is ...
This is a demonstration of why we should add if we have an or probability problem and the combined probability in a or problem will always increase. The next question to ask is what assumptions have we made?; mutually exclusive events, i.e the fish cant be half-in one region and half in another. Is this realistic in the context of the problem?
The fish tank can be rescaled and the divisions changed by dragging the cross-hairs to generate other examples.
There is also a student worksheet for this applet attached.