The factors of 21 are 1, 3, 7 and 21. A factor is a number that can divide evenly into a given number. Because 21 can be divided by numbers other than itself and 1, it is called a composite number.
Every number has 1 and itself as a factor. Numbers that have no other factors are called prime numbers. To determine if 21 has other factors, a person would look to see if it is an even number. Because 21 is not, it cannot be divided by 2. Therefore, the individual would start by dividing by 3. The result is 7, which makes 3 and 7 both factors of 21. Next, 4, 5, and 6 would be tried. None of those can divide evenly into 21, so they are not factors.
1, 3, 7 and 21.
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Because 21 is a factor of 63. 21 is the greatest factor of 21. A larger number doesn't exist.
http://wiki.answers.com/Q/Why_is_21_the_greatest_c...

It's incomplete. 21 isn't prime. 42. 21,2. 7,3,2. 2 x 3 x 7 = 42.
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The GCF is 7.
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