Problem-Solving and Data AnalysisVery hardProbability that two balls match in colour
Problem-Solving and Data Analysis practice question
A bag holds 2 red balls and 2 blue balls. Two balls are drawn at random without replacement. What is the probability that the two balls are the same colour?
Answer: 1/3
P(both red) = 2/4 × 1/3 = 1/6, and P(both blue) is the same, so the total is 1/3.
Common wrong answers
- If you answered 1/2
- 1/2 treats the draws as with replacement.
- If you answered 1/6
- 1/6 counts only one colour.
- If you answered 2/3
- 2/3 is the probability the colours differ.
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