The probability of an event:
0 = the event definitely won’t occur
1 = the event definitely will occur
0.5 = there is a 50/50 chance the event will occur
Probability that event A will happen:
The complement of an event:
The chance the event doesn’t occur--so the complement of drawing a green ball is drawing a ball that isn’t green.
P(event happens) + P(event does not happen) = 1
Mutually exclusive events:
Two events are mutually exclusive if they can’t happen together: P (A and B) = 0
Events A and B (if they are independent events):
P(A and B) = P(A) x P(B)
Events A or B:
A happens, B happens, or both A and B happen.
P (A or B) = P(A) + P(B) – P(A and B)
Events A and B (if A and B are dependent events):
P(A and B) = P(A) x P(B|A)
P(B|A) is the probability that B occurs given that A occurs
(example: the probability of drawing a heart, assuming you already drew a spade).