Best Answer

The GCF of 7 and 21 is 7. Since 7 is a factor of 21, it is automatically the GCF.

Definition: A factor is a divisor - a number that will evenly divide into another number. The greatest common factor of two or more numbers is the largest factor that the numbers have in common.

Method:

One way to determine the common factors and greatest common factor is to find all the factors of the numbers and compare them.

The factors of 7 are 1 and 7.

The factors of 21 are 1, 3, 7, and 21..

The common factors are 1 and 7. Therefore, the greatest common factor is 7.

The GCF is 7.

More answers

The set of factors of 21 = {1, 3, 7, 21}

Comparatively, the set of factors of 7 (owing to the fact that it is a Prime number), = {1, 7}.

Therefore, taking the union of these two sets and looking at its max will give us the highest common factor of the two. The union of these two sets = {1, 7}, which implies that the highest common factor of 21 and 7 = 7.

Q: What is the greatest common factor of 7 and 21?

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The greatest common factor of 21 and 49 is 7.21 = 3 x 7, 49 = 7 x 7 so GCF = 7.

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7 is the greatest common factor of 4942 and 21, or of 49, 42 and 21, if that is what you meant.