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The GCF of 4 and 21 is 1.
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 both numbers have in common.
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 4 are 1, 2, and 4.
The factors of 21 are 1, 3, 7, and 21.
The only common factor is 1. Therefore, the greatest common factor is 1.
The GCF is 1.
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