Problem-Solving and Data AnalysisHardProbability that at least one of two spins is not red
Problem-Solving and Data Analysis practice question
A spinner lands on red with probability 1/4 on each spin, and spins are independent. If it is spun twice, what is the probability that at least one spin is NOT red?
Answer: 15/16
The only way to fail is both spins red: (1/4)(1/4) = 1/16. So P(at least one not red) = 1 − 1/16 = 15/16.
Common wrong answers
- If you answered 1/16
- 1/16 is the probability that BOTH are red — the complement.
- If you answered 3/4
- 3/4 is the probability for one spin only.
- If you answered 9/16
- 9/16 is the probability that both are not red, which is only one way of getting at least one.
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