108 LearnersLast updated on August 5th, 2025
GCF of 100 and 80
The GCF is the largest number that can divide two or more numbers without leaving any remainder. GCF is used to share items equally, to group or arrange items, and to schedule events. In this topic, we will learn about the GCF of 100 and 80.
What is the GCF of 100 and 80?
The greatest common factor of 100 and 80 is 20. The largest divisor of two or more numbers is called the GCF of the number. If two numbers are co-prime, they have no common factors other than 1, so their GCF is 1. The GCF of two numbers cannot be negative because divisors are always positive.
How to find the GCF of 100 and 80?
To find the GCF of 100 and 80, a few methods are described below -
- Listing Factors
- Prime Factorization
- Long Division Method / by Euclidean Algorithm
GCF of 100 and 80 by Using Listing of factors
Steps to find the GCF of 100 and 80 using the listing of factors:
Step 1: Firstly, list the factors of each number
Factors of 100 = 1, 2, 4, 5, 10, 20, 25, 50, 100.
Factors of 80 = 1, 2, 4, 5, 8, 10, 16, 20, 40, 80.
Step 2: Now, identify the common factors of them Common factors of 100 and 80: 1, 2, 4, 5, 10, 20.
Step 3: Choose the largest factor
The largest factor that both numbers have is 20.
The GCF of 100 and 80 is 20.
GCF of 100 and 80 Using Prime Factorization
To find the GCF of 100 and 80 using the Prime Factorization Method, follow these steps:
Step 1: Find the prime factors of each number
Prime Factors of 100: 100 = 2 x 2 x 5 x 5 = 2² x 5²
Prime Factors of 80: 80 = 2 x 2 x 2 x 2 x 5 = 2⁴ x 5
Step 2: Now, identify the common prime factors
The common prime factors are: 2 x 2 x 5 = 2² x 5
Step 3: Multiply the common prime factors 2² x 5 = 4 x 5 = 20.
The Greatest Common Factor of 100 and 80 is 20.
GCF of 100 and 80 Using Division Method or Euclidean Algorithm Method
Find the GCF of 100 and 80 using the division method or Euclidean Algorithm Method. Follow these steps:
Step 1: First, divide the larger number by the smaller number
Here, divide 100 by 80 100 ÷ 80 = 1 (quotient),
The remainder is calculated as 100 − (80 x 1) = 20
The remainder is 20, not zero, so continue the process
Step 2: Now divide the previous divisor (80) by the previous remainder (20)
Divide 80 by 20 80 ÷ 20 = 4 (quotient), remainder = 80 − (20 x 4) = 0
The remainder is zero, the divisor will become the GCF.
The GCF of 100 and 80 is 20.
Common Mistakes and How to Avoid Them in GCF of 100 and 80
Finding GCF of 100 and 80 looks simple, but students often make mistakes while calculating the GCF. Here are some common mistakes to be avoided by the students.
Mistake 1
Listing Incorrect Factors
Students may sometimes list incorrect factors.
For example, while listing factors of 100, students may mention 30, which is incorrect. To avoid this, students should carefully divide the number and list the factors correctly.
Mistake 2
Choosing the wrong common factor
Students may sometimes select the smallest common factor instead of the largest one. To avoid this confusion, students should list all the common factors and find the greatest one.
Mistake 3
Forgetting to include 1 as a factor
Sometimes students may forget 1 as a common factor of the numbers. However, it does not affect the GCF, but it tells about the incomplete understanding of the factors. Students should include 1 as a factor.
Mistake 4
Using Multiples instead of factors
Students confuse between factors and multiples. In that confusion, sometimes they may write multiples instead of factors. To avoid this confusion, students should know the definitions of multiples and factors clearly.
Mistake 5
Assuming GCF is always an even number
Students may assume that GCF of two numbers will always be an even number. But it's not true that a GCF can also be an odd number. To avoid this, students should focus on common factors rather than focusing on even and odd numbers.
Greatest Common Factor of 100 and 80 Examples
Problem 1
A teacher has 100 pencils and 80 erasers. She wants to group them into equal sets, with the largest number of items in each group. How many items will be in each group?
We should find the GCF of 100 and 80 GCF of 100 and 80 2² x 5 = 4 x 5 = 20.
There are 20 equal groups 100 ÷ 20 = 5
80 ÷ 20 = 4
There will be 20 groups, and each group gets 5 pencils and 4 erasers.
Explanation
As the GCF of 100 and 80 is 20, the teacher can make 20 groups. Now divide 100 and 80 by 20. Each group gets 5 pencils and 4 erasers.
Problem 2
A school has 100 red chairs and 80 blue chairs. They want to arrange them in rows with the same number of chairs in each row, using the largest possible number of chairs per row. How many chairs will be in each row?
GCF of 100 and 80 2² x 5 = 4 × 5 = 20.
So each row will have 20 chairs.
Explanation
There are 100 red and 80 blue chairs. To find the total number of chairs in each row, we should find the GCF of 100 and 80. There will be 20 chairs in each row.
Problem 3
A tailor has 100 meters of red ribbon and 80 meters of blue ribbon. She wants to cut both ribbons into pieces of equal length, using the longest possible length. What should be the length of each piece?
For calculating the longest equal length, we have to calculate the GCF of 100 and 80
The GCF of 100 and 80 2² x 5 = 4 × 5 = 20.
The ribbon is 20 meters long.
Explanation
For calculating the longest length of the ribbon first, we need to calculate the GCF of 100 and 80, which is 20. The length of each piece of the ribbon will be 20 meters.
Problem 4
A carpenter has two wooden planks, one 100 cm long and the other 80 cm long. He wants to cut them into the longest possible equal pieces, without any wood left over. What should be the length of each piece?
The carpenter needs the longest piece of wood GCF of 100 and 80
2² x 5 = 4 × 5 = 20.
The longest length of each piece is 20 cm.
Explanation
To find the longest length of each piece of the two wooden planks, 100 cm and 80 cm, respectively. We have to find the GCF of 100 and 80, which is 20 cm. The longest length of each piece is 20 cm.
Problem 5
If the GCF of 100 and ‘a’ is 20, and the LCM is 400. Find ‘a’.
The value of ‘a’ is 80.
Explanation
GCF x LCM = product of the numbers 20 × 400 = 100 × a
8000 = 100a
a = 8000 ÷ 100 = 80
FAQs on the Greatest Common Factor of 100 and 80
1.What is the LCM of 100 and 80?
2.Is 100 divisible by 2?
3.What will be the GCF of any two prime numbers?
4.What is the prime factorization of 80?
5.Are 100 and 80 prime numbers?
6.How can children in United States use numbers in everyday life to understand GCF of 100 and 80?
7.What are some fun ways kids in United States can practice GCF of 100 and 80 with numbers?
8.What role do numbers and GCF of 100 and 80 play in helping children in United States develop problem-solving skills?
9.How can families in United States create number-rich environments to improve GCF of 100 and 80 skills?
Important Glossaries for GCF of 100 and 80
- Factors: Factors are numbers that divide the target number completely. For example, the factors of 20 are 1, 2, 4, 5, 10, and 20.
- Multiple: Multiples are the products we get by multiplying a given number by another. For example, the multiples of 5 are 5, 10, 15, 20, and so on.
- Prime Factors: These are the factors of a number that are prime numbers and divide the given number completely. For example, the prime factors of 30 are 2, 3, and 5.
- Remainder: The value left after division when the number cannot be divided evenly. For example, when 25 is divided by 4, the remainder is 1 and the quotient is 6.
- LCM: The smallest common multiple of two or more numbers is termed LCM. For example, the LCM of 100 and 80 is 400.
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Hiralee Lalitkumar Makwana
About the Author
Hiralee Lalitkumar Makwana has almost two years of teaching experience. She is a number ninja as she loves numbers. Her interest in numbers can be seen in the way she cracks math puzzles and hidden patterns.
Fun Fact
: She loves to read number jokes and games.
