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101 LearnersLast updated on September 18, 2025
GCF of 20 and 80
The GCF is the largest number that can divide two or more numbers without leaving any remainder. GCF is used to share the items equally, to group or arrange items, and schedule events. In this topic, we will learn about the GCF of 20 and 80.
What is the GCF of 20 and 80?
The greatest common factor of 20 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 20 and 80?
To find the GCF of 20 and 80, a few methods are described below
- Listing Factors
- Prime Factorization
- Long Division Method / by Euclidean Algorithm
GCF of 20 and 80 by Using Listing of Factors
Steps to find the GCF of 20 and 80 using the listing of factors:
Step 1: Firstly, list the factors of each number
Factors of 20 = 1, 2, 4, 5, 10, 20.
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 20 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 20 and 80 is 20.
GCF of 20 and 80 Using Prime Factorization
To find the GCF of 20 and 80 using the Prime Factorization Method, follow these steps:
Step 1: Find the prime factors of each number
Prime Factors of 20: 20 = 2 × 2 × 5 = 2² × 5
Prime Factors of 80: 80 = 2 × 2 × 2 × 2 × 5 = 2⁴ × 5
Step 2: Now, identify the common prime factors The common prime factors are: 2 × 2 × 5 = 2² × 5
Step 3: Multiply the common prime factors 2² × 5 = 4 × 5 = 20. The Greatest Common Factor of 20 and 80 is 20.
GCF of 20 and 80 Using Division Method or Euclidean Algorithm Method
Find the GCF of 20 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 80 by 20 80 ÷ 20 = 4 (quotient), The remainder is calculated as 80 − (20×4) = 0
The remainder is zero, the divisor will become the GCF.
The GCF of 20 and 80 is 20.
Common Mistakes and How to Avoid Them in GCF of 20 and 80
Finding GCF of 20 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 20, students may mention 6 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 20 and 80 Examples
Problem 1
A teacher has 20 apples and 80 oranges. 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 GCF of 20 and 80 GCF of 20 and 80 2² × 5 = 4 × 5 = 20.
There are 20 equal groups 20 ÷ 20 = 1 80 ÷ 20 = 4
There will be 20 groups, and each group gets 1 apple and 4 oranges.
Explanation
As the GCF of 20 and 80 is 20, the teacher can make 20 groups.
Now divide 20 and 80 by 20.
Each group gets 1 apple and 4 oranges.
Problem 2
A school has 20 red flags and 80 blue flags. They want to arrange them in rows with the same number of flags in each row, using the largest possible number of flags per row. How many flags will be in each row?
GCF of 20 and 80 2² × 5 = 4 × 5 = 20. So each row will have 20 flags.
Explanation
There are 20 red and 80 blue flags.
To find the total number of flags in each row, we should find the GCF of 20 and 80.
There will be 20 flags in each row.
Problem 3
A tailor has 20 meters of red fabric and 80 meters of blue fabric. She wants to cut both fabrics 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 20 and 80
The GCF of 20 and 80 2² × 5 = 4 × 5 = 20.
The fabric is 20 meters long.
Explanation
For calculating the longest length of the fabric, first, we need to calculate the GCF of 20 and 80, which is 20.
The length of each piece of fabric will be 20 meters.
Problem 4
A carpenter has two wooden planks, one 20 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 20 and 80 2² × 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, 20 cm and 80 cm, respectively.
We have to find the GCF of 20 and 80, which is 20 cm.
The longest length of each piece is 20 cm.
Problem 5
If the GCF of 20 and ‘b’ is 20, and the LCM is 160, find ‘b’.
The value of ‘b’ is 80.
Explanation
GCF × LCM = product of the numbers
20 × 160 = 20 × b
3200 = 20b
b = 3200 ÷ 20 = 80
FAQs on the Greatest Common Factor of 20 and 80
1.What is the LCM of 20 and 80?
2.Is 20 divisible by 5?
3.What will be the GCF of any two prime numbers?
4.What is the prime factorization of 80?
5.Are 20 and 80 prime numbers?
Important Glossaries for GCF of 20 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, 25, 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 20 are 2 and 5.
- Remainder: The value left after division when the number cannot be divided evenly. For example, when 20 is divided by 6, the remainder is 2 and the quotient is 3.
- LCM: The smallest common multiple of two or more numbers is termed LCM. For example, the LCM of 20 and 80 is 80.
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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.
