Addition Rule For Probabilities
Definition of 'Addition Rule For Probabilities'
A statistical property that states the probability of one and/or two events occurring at the same time is equal to the probability of the first event occurring, plus the probability of the second event occurring, minus the probability that both events occur at the same time.
Mathematically, this property is denoted by: 

Investopedia explains 'Addition Rule For Probabilities'
For example, assume we wish to determine the probability of drawing a king and/or a queen out of a deck of cards on only one draw. Using the addition rule for probabilities, we get the following: P(King) = 4/52, P(Queen) = 4/52, and P(King and Queen) = 0.
Since it is impossible to draw both a king and queen on the same draw, we can conclude that the probability of drawing either a king or queen from a deck of cards is 8/52, or about 15.4%. 