The greatest common factor (GCF) is the greatest number that is a factor of two or more numbers. The first step for finding a GCF is to break down each number into its prime factors. We will write down all of the prime factors that are common between the given numbers. Notice that 7 is a prime number, which means it is its own prime factorization.
Given number
Prime Factorization
Common Prime Factors
7
7
7
28
2* 2* 7
2, 2, 7
For the given values, the only common prime factor is 7. This means that the GCF for this pair of numbers is 7.