MathProblem-Solving & Data AnalysisHigh frequency
SAT Math: Basic Probability Rules
28+ practice questions in Praczo
What you need to know
The concept, explained
- 1
Probability = (favorable outcomes) / (total outcomes). Must be between 0 and 1.
- 2
P(A or B) = P(A) + P(B) − P(A and B). Subtract the overlap to avoid double-counting.
- 3
P(A and B) for independent events = P(A) × P(B).
- 4
Complementary probability: P(not A) = 1 − P(A). Often faster to compute the complement.
- 5
Be careful with replacement vs. without replacement — without replacement changes the probability on each draw.
Common mistakes
- ✗ Adding probabilities for "and" events instead of multiplying: P(A and B) ≠ P(A) + P(B).
- ✗ Forgetting to subtract the intersection in "or" problems: P(A or B) is not simply P(A) + P(B) when A and B can overlap.
Try a sample question
SAT-style practice
A bag has 4 red and 6 blue marbles. If one marble is drawn at random, what is the probability it is NOT red?
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