Here are some probability formulas:
P(A): The probability of an event
n(A): The number of favorable outcomes
n(S): The total number of events in the sample space
P(A or B): P(A) + P(B) – P(A and B)
P(A and B): P(A) x P(B|A) = P(B) x P(A|B)
P(A|B): P(A and B) / P(B)
P(B|A): P(A and B) / P(A)
P(not A): 1 - P(A)
P(A ∩ B): P(A)⋅P(B) (in case of independent events)
P(AC) + P(A): 1
P(A∪B): P(A) + P(B) – P(A∩B)