List all of the prime numbers less than 20 in numerical order

19, 17, 13, 11, 7, 5, 3, 2

A prime number is a number only divisible by itself and 1 (this excludes decimals and fractions)

surprise, 1 is not prime because it must be divisible by itself and 1 and the factors of 1 are 1 and 1 so 1 is not prime

2 is prime because it is the first even counting number and since all even numbers after 2 are divisble by 2 you can cross out all odd numbers after 2

3 is prime because it is the first odd counting number that satisfies the condition

4 is not because it is even

5 is prime

6 is not because it is even

7 is prime

8 is not because it is even

9 is not

10 is not because it is even

11 is prime

12 is not because it is even

13 is prime

14 is not because it is even

15 is not

16 is not because it is even

17 is prime

18 is not because it is even

19 is prime

20 is not because it is even

so the numbers are 2,3,5,7,11,13,17,19