What is probability and how do you calculate it?

Probability measures the likelihood of an event occurring, expressed as a number between 0 (impossible) and 1 (certain). Formula: P(event) = favorable outcomes / total outcomes. Example: Rolling a 6 on a die → P = 1/6 ≈ 0.167. Key rules: P(A or B) = P(A) + P(B) - P(A and B) for union. P(A and B) = P(A) × P(B) for independent events. Complementary: P(not A) = 1 - P(A).