Picking a prime — Probability Aptitude Question
Picking a prime: A number is picked at random from 1 to 20. What is the probability that it is prime? Answer and worked solution on CodeKairo.
- Topic: Probability
- Section: Quantitative Aptitude
- Difficulty: Easy
- Time target: 50 seconds
Question
A number is picked at random from 1 to 20. What is the probability that it is prime?
Options
- 1/5
- 2/5
- 9/20
- 1/2
Show the answer and the worked solution
Correct answer: B. 2/5
Worked solution
- Primes from 1 to 20: 2, 3, 5, 7, 11, 13, 17, 19 — eight of them.
- Probability = 8/20 = 2/5.
Answer: 2/5.
The idea
Remember that 1 is not prime and 2 is. Miscounting by one is the only real risk here.
Shortcut. Four primes among the single digits (2, 3, 5, 7) and four in the teens (11, 13, 17, 19): 8/20 = 2/5.
Common trap. "9/20" — counting 1 as a prime, which makes nine; 1 has only one divisor and is not prime.