Probability in Plain English
Probability is just favorable outcomes ÷ total outcomes — a number between 0 (impossible) and 1 (certain). A coin flip is 0.5, a specific lottery ticket is roughly 1 in 300 million. Everything else is built from that one idea.
The Four Rules on This Page
- Simple: P(A) = favorable ÷ total.
- Complement: P(not A) = 1 − P(A) — sometimes the “not” is easier to compute.
- AND (independent): multiply — P(A)×P(B). Two coin flips both heads: 0.5 × 0.5 = 0.25.
- OR (independent): add, then subtract the overlap — P(A)+P(B)−P(A)×P(B).
The Classic Trap
People add probabilities for AND (“10% and 10% = 20%”) — wrong. Two independent 10% events both happening is 0.1 × 0.1 = 1%, not 20%. Multiplication shrinks; addition grows. When in doubt, the calculator keeps you honest.
Counting the outcomes first? Our factorial calculator handles permutations and combinations.