Solution: The GCF of 5 and 15 is 5MethodsHow to find the GCF of 5 and 15 using Prime FactorizationOne way to find the GCF of 5 and 15 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 5?What are the Factors of 15?Here is the prime factorization of 5:515^151And this is the prime factorization of 15:31×513^1 × 5^131×51When 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 5 and 15 by multiplying all the matching prime factors to get a GCF of 5 and 15 as 25:Thus, the GCF of 5 and 15 is: 25How to Find the GCF of 5 and 15 by Listing All Common FactorsThe first step to this method of finding the Greatest Common Factor of 5 and 15 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, 5 and 15:Factors of 5: 1, 5Factors of 15: 1, 3, 5, 15When you compare the two lists of factors, you can see that the common factor(s) are 1, 5. Since 5 is the largest of these common factors, the GCF of 5 and 15 would be 5.Find the GCF of Other Number PairsWant more practice? Try some of these other GCF problems:What is the GCF of 55 and 70?What is the GCF of 105 and 25?What is the GCF of 99 and 18?What is the GCF of 119 and 98?What is the GCF of 1 and 80?