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

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

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

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