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# What what is the greatest common factor 21 and 28?

Updated: 8/20/2019

Wiki User

11y ago

The GCF of 21 and 28 is 7.

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

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

The factors of 28 are 1, 2, 4, 7, 14, and 28.

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

The greatest common factor can also be calculated by identifying the common prime factors and multiplying them together.

The prime factors of 21 are 3 and 7.

The prime factors of 28 are 2, 2, and 7.

The prime factors in common are a single 7, so the greatest common factor is 7.
7
7
There is neither a greatest common factor nor common factors of a single number, such as 2821, because there cannot be any form of common factor without two or more numbers to compare. Common factors are factors that the numbers being compared have in common. The greatest common factor is the largest factor that all the numbers being compared have in common. Thus, since there are not two or more numbers to compare, there are neither common factors nor a greatest common factor.

Wiki User

5y ago

Wiki User

6y ago

GCF(21, 28) = 7.

Wiki User

11y ago

7

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