The probability of an event is determined by the following formula:
P(E)= #Success/Possible outcomes
For example, the probability when rolling a fair die and getting a 6 is:
P(6)= 1/6
That is there is 1 success with 6 outcomes
The probability of an event not happening is:
P(not happening) = 1 - P(E)
The probability of rolling a fair die and not getting a 6 is:
P(not 6) = 1- (1/6)= 5/6
Probability of mutually exclusive events
Two events are mutually exclusive when they can NOT happen at the same time
For example:
When I rolled a die, either you get a 2 or a 3
When you toss a coin, you either get heads or tails
The formula for mutually exclusive events is:
P(A or B) = P(A) + P(B)