**Complementary event or ‘not event’** : The set A′ or S – A

**Event A or B:** The set A ∪ B

**Event A and B:** The set A ∩ B

**Event A and not B:** The set A – B

**Mutually exclusive event:** A and B are mutually exclusive if A ∩ B = φ

**Equally likely outcomes:** All outcomes with equal probability

**Probability of an event:** For a finite sample space with equally likely outcomes

Probability of an event \( P(A) = \frac{n(A)}{n(S)} \), where n(A) = number of elements in the set A,

n(S) = number of elements in the set S.

If A and B are any two events, then

P(A or B) = P(A) + P(B) – P(A and B) equivalently,

P(A ∪ B) = P(A) + P(B) – P(A ∩ B)

If A and B are mutually exclusive, then P(A or B) = P(A) + P(B)

If A is any event, then P(not A) = 1 – P(A)