Probability ranges from 0 (impossible) to 1 (certain). P(A) = favourable outcomes ÷ total outcomes.
P(A AND B) — both events occur (independent events): P(A) × P(B)
P(A OR B) — either event occurs: P(A) + P(B) − P(A AND B)
At least once in n trials: 1 − P(not A)ⁿ
What is the probability of rolling a 6 at least once in 4 rolls?
P(6) = 1/6. P(not 6) = 5/6. P(at least one 6 in 4 rolls) = 1 − (5/6)⁴ = 51.8%.
Does this assume independent events?
Yes. P(A AND B) = P(A) × P(B) only holds when A and B are independent (one doesn't affect the other).