Problem-Solving and Data AnalysisMediumTwo defective bulbs in a row
Problem-Solving and Data Analysis practice question
A box holds 10 light bulbs, 3 of which are faulty. Two bulbs are taken out at random without replacement. What is the probability that both are faulty?
Answer: 1/15
3/10 × 2/9 = 6/90 = 1/15.
Common wrong answers
- If you answered 9/100
- 9/100 treats the draws as with replacement.
- If you answered 3/10
- 3/10 is the probability of one faulty bulb.
- If you answered 1/5
- 1/5 adds 3/10 and 2/9 wrongly, or halves 3/10 and errs.
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