Solution: The GCF of 111 and 21 is 3MethodsHow to find the GCF of 111 and 21 using Prime FactorizationOne way to find the GCF of 111 and 21 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 111?What are the Factors of 21?Here is the prime factorization of 111:31×3713^1 × 37^131×371And this is the prime factorization of 21:31×713^1 × 7^131×71When you compare the prime factorization of these two numbers, you can see that there are matching prime factors. You can now find the Greatest Common Factor of 111 and 21 by multiplying all the matching prime factors to get a GCF of 111 and 21 as 9:Thus, the GCF of 111 and 21 is: 9How to Find the GCF of 111 and 21 by Listing All Common FactorsThe first step to this method of finding the Greatest Common Factor of 111 and 21 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, 111 and 21:Factors of 111: 1, 3, 37, 111Factors of 21: 1, 3, 7, 21When you compare the two lists of factors, you can see that the common factor(s) are 1, 3. Since 3 is the largest of these common factors, the GCF of 111 and 21 would be 3.Find the GCF of Other Number PairsWant more practice? Try some of these other GCF problems:What is the GCF of 100 and 144?What is the GCF of 51 and 42?What is the GCF of 50 and 10?What is the GCF of 106 and 113?What is the GCF of 150 and 25?