9 is not a prime number. It can be divided by 3 as well as 1 and 9. The prime numbers below 20 are: 2, 3, 5, 7, 11, 13, 17, 19. Don't forget: the number 1 is not thought of as a prime number.

But it takes a lot of time and computing power to try all the prime numbers up to a hundred or more digits long. This is how encryption stays a hair’s breadth ahead of hackers, and it’s one ...