Solution: The GCF of 20 and 5 is 5
Methods
How to find the GCF of 20 and 5 using Prime Factorization
One way to find the GCF of 20 and 5 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 20?
What are the Factors of 5?
Here is the prime factorization of 20:
2
2
×
5
1
2^2 × 5^1
2 2 × 5 1
And this is the prime factorization of 5:
5
1
5^1
5 1
When 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 20 and 5 by multiplying all the matching prime factors to get a GCF of 20 and 5 as 25:
Thus, the GCF of 20 and 5 is: 25
How to Find the GCF of 20 and 5 by Listing All Common Factors
The first step to this method of finding the Greatest Common Factor of 20 and 5 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, 20 and 5:
Factors of 20: 1, 2, 4, 5, 10, 20
Factors of 5: 1, 5
When 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 20 and 5 would be 5.
Find the GCF of Other Number Pairs
Want more practice? Try some of these other GCF problems:
What is the GCF of 124 and 138?
What is the GCF of 82 and 43?
What is the GCF of 23 and 49?
What is the GCF of 4 and 116?
What is the GCF of 39 and 31?