# Factors and Primes

A factor is a whole number that fits evenly into another whole number. For example, 3 is a factor of 9, because you can fit 3 evenly three times into 9: 3 x 3 = 9.

Four is not a factor of 9, because there is no way to fit fours evenly into 9: 4 x 2 = 8, leaving one left over, and 4 x 3 = 12, which is three too many.

To find all the factors of a number, you need to check all of the smaller numbers to see whether they fit evenly into that number. There isn't a system or a trick to do it. On the other hand, you can certainly eliminate some choices - if you know that 2 isn't a factor of 27, then you can see that no multiples of 2 will be factors of 27. So you don't have to try any even number. If a number ends in 0 or 5, then five is one of its factors, but if it doesn't end in 0 or 5, then five is not one of its factors.

Some numbers have no factors other than one and themselves. We call these numbers *prime numbers*. For example, 1, 3, 5, 7, and 11 are all prime numbers. Larger numbers can be prime too: 47 is prime, and 997 is also prime.

You need to use factors to find common denominators of fractions in order to add fractions.

## More about common denominators

More about fractions

## Bibliography and further reading about numbers:

## More about common denominators

More about fractions

More about Math

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