What is the key word that indictes that the Addition Rule for Probabilities will be used?
The key word for events with probabilities that can be added is mutually exclusive.
If two events,
An explanation and intuitive justification of this rule is a similarity of a concept of probability and a concept of an area of a flat figure on a plane. Mutually exclusive events are similar to two figures on a plane that do not have any points in common. Then it's obvious that the area of both figures (the probability of one event or another) equals to a sum of areas of these two figures (equals to a sum of probabilities of individual events).