Solution: The GCF of 28 and 139 is 1MethodsHow to find the GCF of 28 and 139 using Prime FactorizationOne way to find the GCF of 28 and 139 is to compare the prime factorization of each number. To find the prime factorization, you can follow the instructions for each number here:What are the Factors of 28?What are the Factors of 139?Here is the prime factorization of 28:22×712^2 × 7^122×71And this is the prime factorization of 139:1391139^11391When you compare the prime factorization of these two numbers, you can see that there are no matching prime factors. When this is the case, it means that there are no common factors between these two numbers. As a result, the GCF of 28 and 139 is 1.Thus, the GCF of 28 and 139 is: 1How to Find the GCF of 28 and 139 by Listing All Common FactorsThe first step to this method of finding the Greatest Common Factor of 28 and 139 is to find and list all the factors of each number. Again, you can see how this is done by looking at the “Factors of” articles that are linked to above.Let’s take a look at the factors for each of these numbers, 28 and 139:Factors of 28: 1, 2, 4, 7, 14, 28Factors of 139: 1, 139When you compare the two lists of factors, you can see that the only common factor is 1. So, in this case, the GCF of 28 and 139 is 1.Find the GCF of Other Number PairsWant more practice? Try some of these other GCF problems:What is the GCF of 70 and 141?What is the GCF of 139 and 72?What is the GCF of 91 and 35?What is the GCF of 46 and 32?What is the GCF of 17 and 7?