Prime numbers are those numbers (> 1) that cannot be divided by
any number except themselves and one.
The Greek Eratosthenes created a method to find out these prime
numbers, although it only worked over a limited range:
1) Write out the numbers from 1 to 100 in ten rows of 10.
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20
...........................................
...........................................
91 92 93 94 95 96 97 98 99 100
2) Cross off number 1, because all primes are greater than 1.
3) Number 2 is a prime, so we can keep it, but we need to cross
off the multiples of 2 (i.e. even numbers).
4) Number 3 is also a prime, so again we keep it and cross off the
multiples of 3.
5) The next number left is 5 (because four has been crossed off),
so we keep it and cross of the multiples of this number.
6) The final number left in the first row is number 7, so cross
off its multiples.
7) You have finished. All of the "surviving" numbers
(colored in white below) on your grid are prime numbers.
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