Prime numbers between 1 and 20
There are 8 prime numbers between 1 and 20: 2, 3, 5, 7, 11, 13, 17, 19.
How to work it out
- 1
A prime has exactly two factors, 1 and itself. To list them, cross out multiples of 2, then 3, then 5, and so on up to ā20 ā 4.5 (the sieve of Eratosthenes). What is left are the primes.
- 2
That leaves 8 primes between 1 and 20, with a total of 77.
- 3
The biggest gap between neighbouring primes in this range is 4, after 7 (the next prime is 11). There are 4 twin prime pairs (primes 2 apart): (3, 5), (5, 7), (11, 13), (17, 19).
How many primes in each part
| Range | Primes | Share of the numbers |
|---|---|---|
| 1 to 10 | 4 | 40% |
| 11 to 20 | 4 | 40% |
Questions
How many prime numbers are there between 1 and 20?
8.
What is the biggest prime number below 20?
19.
What is the first prime number after 0?
2.