addition rule

a simple principle of probability theory is that the probability of either of two different outcomes occurring is the sum of the separate probabilities for those two different events (Figure A.2). So, the probability of a die landing 3 is 1 divided by 6 (i.e. 0.167) and the probability of a die landing 5 is 1 divided by 6 (i.e. 0.167 again). The probability of getting either a 3 or a 5 when tossing a die is the sum of the two separate probabilities (i.e. 0.167 + 0.167 = 0.333). Of course, the probability of getting any of the numbers from 1 to 6 spots is 1.0 (i.e. the sum of six probabilities of 0.167).