Probability is the likelihood or chance of an event occurring.
Some Concepts
The probability of a certain event occurring can be represented by P(A). The probability of a different event occurring can be written as P(B). Therefore, for two events A and B,
P(A) + P(B) - P(A ∩ B) = P(A ∪ B)
Events A and B are mutually exclusive if they have no events in common. If two events are mutually exclusive,
P(A) + P(B) = P(A ∪ B)
Two events are independent if (and only if)
(A ∩ B) = P(A)P(B)
Conditional probability is the probability of an event occurring, given that another event has occurred.
P(A|B) means the probability of A occurring, given that B has occurred.
For two events A and B,
P(A ∩ B) = P(A|B)P(B)
P(A ∩ B) = P(B|A)P(A)