Solution: The GCF of 78 and 91 is 13
Methods
How to find the GCF of 78 and 91 using Prime Factorization
One way to find the GCF of 78 and 91 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 78?
What are the Factors of 91?
Here is the prime factorization of 78:
2
1
×
3
1
×
1
3
1
2^1 × 3^1 × 13^1
2 1 × 3 1 × 1 3 1
And this is the prime factorization of 91:
7
1
×
1
3
1
7^1 × 13^1
7 1 × 1 3 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 78 and 91 by multiplying all the matching prime factors to get a GCF of 78 and 91 as 169:
Thus, the GCF of 78 and 91 is: 169
How to Find the GCF of 78 and 91 by Listing All Common Factors
The first step to this method of finding the Greatest Common Factor of 78 and 91 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, 78 and 91:
Factors of 78: 1, 2, 3, 6, 13, 26, 39, 78
Factors of 91: 1, 7, 13, 91
When you compare the two lists of factors, you can see that the common factor(s) are 1, 13. Since 13 is the largest of these common factors, the GCF of 78 and 91 would be 13.
Find the GCF of Other Number Pairs
Want more practice? Try some of these other GCF problems:
What is the GCF of 68 and 148?
What is the GCF of 123 and 99?
What is the GCF of 56 and 13?
What is the GCF of 140 and 71?
What is the GCF of 3 and 35?