# Prime and composite numbers

A number is **prime** if it has exactly two factors: 1 and itself. If the number is not prime it is **composite**.

Example: 20 is composite and 13 is prime.

Exercise: Do the next table:

Cross out the 2's, 3's, 4's and 6's columns except numbers 2 and 3.

Find the 5's multiples and cross them out; notice that they are in a straight line.

Do the same with 7, 11,... , are they in a straight line too? At the end, you get the prime number list.

Prime numbers:{2, 3, 5, 7, 11, 13, 17, 19, …}

**Exercise**: find out if the following numbers are prime or composite ones: 91, 97, 121, 169 and 173.

Solution: prime numbers={97,173}; composite numbers={91,121,169}

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