Answer:
{2, 3, 5, 7, 11, 13, 17, 19}
Explanation:
You want the prime numbers between 1 and 20.
Prime
A prime number is whole number divisible only by 1 and itself.
2 is the first prime. It eliminates all even numbers from consideration.
3 is the second prime. It eliminates multiples of 3 from consideration, so we can exclude 9 and 15. (The other multiples of 3 in that range are even.)
The numbers that have not been eliminated so far are all prime. The full list is ...
{2, 3, 5, 7, 11, 13, 17, 19}
__
Additional comment
When testing a number for primality, one only needs to check prime divisors up to the square root of the number. Here, all numbers less than 25 can be tested for primality by checking their divisibility by 2 and 3.