Either one or the other — Probability Aptitude Question
Either one or the other: P(A) = 0.4, P(B) = 0.5, and P(A and B) = 0.2. What is P(A or B)? Answer and worked solution on CodeKairo.
- Topic: Probability
- Section: Quantitative Aptitude
- Difficulty: Easy
- Time target: 45 seconds
Question
P(A) = 0.4, P(B) = 0.5, and P(A and B) = 0.2. What is P(A or B)?
Options
- 0.6
- 0.7
- 0.8
- 0.9
Show the answer and the worked solution
Correct answer: B. 0.7
Worked solution
- P(A or B) = 0.4 + 0.5 − 0.2 = 0.7.
Answer: 0.7.
The idea
Subtract the overlap once. Omitting it gives 0.9, which is offered as the trap.
Shortcut. Split into pieces: only A = 0.2, only B = 0.3, both = 0.2; together they make 0.7.
Common trap. "0.9" — adding 0.4 + 0.5 and so counting the 0.2 overlap twice.