Prime numbers are numbers that have only one factor pair; 1 and themselves.
Let's start from 21.
21 = 1 x 21, 3 x 7.
This has two factor pairs, this is not prime.
Let's try 22.
22 = 1 x 22, 2 x 11.
This also has two factor pairs, this is not prime.
Let's try 23.
1 x 23.
This is a prime number, as it only has one factor pair; 1 and itself.
23 is your prime number.
I hope this helps!
