Drawing a defective bulb — Probability Aptitude Question
Drawing a defective bulb: A box has 20 bulbs, of which 4 are defective. One bulb is drawn at random. Answer and worked solution on CodeKairo.
- Topic: Probability
- Section: Quantitative Aptitude
- Difficulty: Easy
- Time target: 40 seconds
Question
A box has 20 bulbs, of which 4 are defective. One bulb is drawn at random. What is the probability that it is not defective?
Options
- 1/5
- 3/5
- 4/5
- 9/10
Show the answer and the worked solution
Correct answer: C. 4/5
Worked solution
- Good bulbs = 16 of 20.
- Probability = 16/20 = 4/5.
Answer: 4/5.
The idea
Take the complement when it is easier: 1 − 4/20 = 4/5 gives the same result in one step.
Shortcut. 4 in 20 is 1 in 5 defective, so 4 in 5 are good: 4/5.
Common trap. "1/5" — answering with the probability of a defective bulb, 4/20, when the question asks for one that is not defective.