what does the addition rule for probabilities tell you 
to illustrate the first rule in the addition rule for probabilities consider a die with six sides and the chances of rolling either a 3 or a 6 since the chances of rolling a 3 are 1 in 6 and the chances of rolling a 6 are also 1 in 6 the chance of rolling either a 3 or a 6 is to illustrate the second rule consider a class in which there are 9 boys and 11 girls at the end of the term 5 girls and 4 boys receive a grade of b if a student is selected by chance what are the odds that the student will be either a girl or a b student since the chances of selecting a girl are 11 in 20 the chances of selecting a b student are 9 in 20 and the chances of selecting a girl who is a b student are 5 20 the chances of picking a girl or a b student are in reality the two rules simplify to just one rule the second one that s because in the first case the probability of two mutually exclusive events both happening is 0 in the example with the die it s impossible to roll both a 3 and a 6 on one roll of a single die so the two events are mutually exclusive mutual exclusivitymutually exclusive is a statistical term describing two or more events that cannot coincide it is commonly used to describe a situation where the occurrence of one outcome supersedes the other for a basic example consider the rolling of dice you cannot roll both a five and a three simultaneously on a single die furthermore getting a three on an initial roll has no impact on whether or not a subsequent roll yields a five all rolls of a die are independent events 