Factors, primes and multiples

A factor of a number divides it exactly. To find all the factors of 12, look for pairs that multiply to 12: 1 × 12, 2 × 6 and 3 × 4. So the factors of 12 are 1, 2, 3, 4, 6 and 12. Test the numbers in order, 1, 2, 3 and so on, and stop when the pairs start to repeat. An array also shows a factor pair: 12 dots can make 1 row of 12, 2 rows of 6 or 3 rows of 4. A prime number has exactly two factors, 1 and itself: 2, 3, 5, 7, 11, 13, 17, 19, 23 and 29 are prime. A composite number has three or more factors, like 12. The number 1 has only one factor, so it is neither prime nor composite. Be careful: 2 is the only even prime number.