Primes below 50 — Number System Aptitude Question
Primes below 50: How many prime numbers are there between 1 and 50? Answer and worked solution on CodeKairo.
- Topic: Number System
- Section: Quantitative Aptitude
- Difficulty: Easy
- Time target: 45 seconds
Question
How many prime numbers are there between 1 and 50?
Options
- 13
- 14
- 15
- 16
Show the answer and the worked solution
Correct answer: C. 15
Worked solution
- The primes are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47.
- Counting them gives 15.
Answer: 15.
The idea
Worth memorising: there are 15 primes below 50 and 25 below 100. The count by decade is 4, 4, 2, 2, 3, 2, 2, 3, 2, 1.
Shortcut. Beyond 2 and 3 every prime is 6k ± 1. Below 50 there are 16 such numbers; striking 25, 35 and 49 leaves 13, and adding 2 and 3 gives 15.
Common trap. 14 — scanning only the odd numbers and missing 2, the one even prime.