100 LearnersLast updated on August 5th, 2025
GCF of 21 and 15
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 schedule events. In this topic, we will learn about the GCF of 21 and 15.
What is the GCF of 21 and 15?
The greatest common factor of 21 and 15 is 3. The largest divisor of two or more numbers is called the GCF of the numbers. 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 21 and 15?
To find the GCF of 21 and 15, a few methods are described below
- Listing Factors
- Prime Factorization
- Long Division Method / by Euclidean Algorithm
GCF of 21 and 15 by Using Listing of factors
Steps to find the GCF of 21 and 15 using the listing of factors:
Step 1: Firstly, list the factors of each number
Factors of 21 = 1, 3, 7, 21.
Factors of 15 = 1, 3, 5, 15.
Step 2: Now, identify the common factors of them Common factors of 21 and 15: 1, 3.
Step 3: Choose the largest factor:
The largest factor that both numbers have is 3.
The GCF of 21 and 15 is 3.
GCF of 21 and 15 Using Prime Factorization
To find the GCF of 21 and 15 using the Prime Factorization Method, follow these steps:
Step 1: Find the prime factors of each number.
Prime Factors of 21: 21 = 3 x 7
Prime Factors of 15: 15 = 3 x 5
Step 2: Now, identify the common prime factors. The common prime factor is: 3
Step 3: Multiply the common prime factors 3 = 3. The Greatest Common Factor of 21 and 15 is 3.
GCF of 21 and 15 Using Division Method or Euclidean Algorithm Method
Find the GCF of 21 and 15 using the division method or Euclidean Algorithm Method. Follow these steps:
Step 1: First, divide the larger number by the smaller number
Here, divide 21 by 15 21 ÷ 15 = 1 (quotient),
The remainder is calculated as 21 − (15×1) = 6
The remainder is 6, not zero, so continue the process
Step 2: Now divide the previous divisor (15) by the previous remainder (6)
Divide 15 by 6 15 ÷ 6 = 2 (quotient), remainder = 15 − (6×2) = 3
Step 3: Divide the previous divisor (6) by the remainder (3) 6 ÷ 3 = 2 (quotient), remainder = 6 − (3×2) = 0
The remainder is zero, the divisor will become the GCF.
The GCF of 21 and 15 is 3.
Common Mistakes and How to Avoid Them in GCF of 21 and 15
Finding GCF of 21 and 15 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 21, students may mention 2 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 the GCF of two numbers will always be an even number. But it's not true; 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 21 and 15 Examples
Problem 1
A gardener has 21 tulips and 15 roses. She wants to plant them in equal groups with the largest number of flowers in each group. How many flowers will be in each group?
We should find the GCF of 21 and 15 GCF of 21 and 15 3.
There are 3 equal groups
21 ÷ 3 = 7
15 ÷ 3 = 5
There will be 3 groups, and each group gets 7 tulips and 5 roses.
Explanation
As the GCF of 21 and 15 is 3, the gardener can make 3 groups. Now divide 21 and 15 by 3. Each group gets 7 tulips and 5 roses.
Problem 2
A chef has 21 apples and 15 oranges. He wants to pack them in boxes with the same number of fruits in each box, using the largest possible number of fruits per box. How many fruits will be in each box?
GCF of 21 and 15 3. So each box will have 3 fruits.
Explanation
There are 21 apples and 15 oranges. To find the total number of fruits in each box, we should find the GCF of 21 and 15. There will be 3 fruits in each box.
Problem 3
A tailor has 21 meters of silk and 15 meters of cotton. 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 21 and 15 The GCF of 21 and 15 3. The fabric is 3 meters long.
Explanation
For calculating the longest length of the fabric, first we need to calculate the GCF of 21 and 15, which is 3. The length of each piece of fabric will be 3 meters.
Problem 4
A carpenter has two wooden planks, one 21 cm long and the other 15 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 21 and 15 3. The longest length of each piece is 3 cm.
Explanation
To find the longest length of each piece of the two wooden planks, 21 cm and 15 cm, respectively.
We have to find the GCF of 21 and 15, which is 3 cm.
The longest length of each piece is 3 cm.
Problem 5
If the GCF of 21 and ‘b’ is 3, and the LCM is 105, find ‘b’.
The value of ‘b’ is 15.
Explanation
GCF x LCM = product of the numbers
3 × 105 = 21 × b
315 = 21b
b = 315 ÷ 21 = 15
FAQs on the Greatest Common Factor of 21 and 15
1.What is the LCM of 21 and 15?
2.Is 21 divisible by 7?
3.What will be the GCF of any two prime numbers?
4.What is the prime factorization of 15?
5.Are 21 and 15 prime numbers?
6.How can children in United States use numbers in everyday life to understand GCF of 21 and 15?
7.What are some fun ways kids in United States can practice GCF of 21 and 15 with numbers?
8.What role do numbers and GCF of 21 and 15 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 21 and 15 skills?
Important Glossaries for GCF of 21 and 15
- Factors: Factors are numbers that divide the target number completely. For example, the factors of 21 are 1, 3, 7, and 21.
- Multiple: Multiples are the products we get by multiplying a given number by another. For example, the multiples of 3 are 3, 6, 9, 12, 15, 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 21 are 3 and 7.
- Remainder: The value left after division when the number cannot be divided evenly. For example, when 21 is divided by 4, the remainder is 1 and the quotient is 5.
- LCM: The smallest common multiple of two or more numbers is termed LCM. For example, the LCM of 21 and 15 is 105.
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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.
