BrightChamps Logo
Hamburger Menu Icon for BrightChamps Website Navigation
Login
Creative Math Ideas Image
Live Math Learners Count Icon281 Learners

Last updated on February 13th, 2025

Math Whiteboard Illustration

Is 21 a Prime Number?

Professor Greenline Explaining Math Concepts

Prime numbers are special because they only have two divisors: 1 and themselves. We see them closer to the home, just in ATM pins. From here we start playing with prime numbers, and the reason 21 isn’t a prime.

Is 21 a Prime Number? for Qatari Students
Professor Greenline from BrightChamps

Is 21 a Prime Number?

21 is not a prime number. But according to what we are saying, why is it not a prime number? Let's understand that, firstly, the number set is of two parts:

 

  • Prime number

 

  • composite numbers

 

And using one of the methods we're going to introduce you to, we will find that indeed 21 is not a prime.

 

is 21 a prime number
 

Professor Greenline from BrightChamps

Why Is 21 Not a Prime Number?

We could find that a number is prime if it has 2 factors only, 1 and the number itself, and only 2 divisors. It means that a number, not satisfying these conditions, cannot be a prime number.


However, there are pretty simple methods to determine whether a number is prime or not:

 

  • Count Divisor Method

 

  • Divisibility Test

 

  • Prime Number Table

 

  • Prime Factorization
     
Professor Greenline from BrightChamps

Using the Counting Divisors Method

The name implies: We’re counting how many divisors a given number has, and then saying that number is prime. It is easy to learn. With that said, let’s see what steps are there in this method.

 

  • First is to count how many the number holds in divisors.

 

  • We can then see that 21 has more than 2 divisors: 1,3,7 and 21.

 

We can see that number 21 has 4 divisors. Thus, we see that it doesn’t fulfill the criteria needed for a prime number. And so it is not a prime number.
 

Professor Greenline from BrightChamps

Using the Divisibility Test Method

In this case, we'll see if 21 can be divided into any other number. Now if it gets divided then that is not a prime number. So let’s check for 2,3,5 and 7.

 

  • Since 21 is an even number, we can say that 21 is divisible by 2.

 

  • Since the sum of the digits is 3, 3 is divisible by 3 as 3 is a multiple of 3. Hence, it is divisible by 3.

 

  • 21 is not ending with a 0 or 5. Therefore, it is not divisible by 5.

 

  • To see if 21 is divisible by 7, we can use a simple trick. First, we look at the last digit, which is 1. We double that 1 (1 × 2 = 2) and then subtract it from the other digit, which is 1. So, we do 2–2 = 0. 0 is not a multiple of 7,but when the difference is 0, the number will be divisible by 7. Hence, 21 is divisible by 7.

 

We know 21 can be divided by two of the above-mentioned numbers, therefore it is not a prime number.
 

Professor Greenline from BrightChamps

Using A Prime Number Chart

Here, we will be using a chart that will contain all the prime numbers between 1 and 100. If the given number appears in the list, it is not a prime number. 
 

 

 

 

We can see that 21 does not appear here. So, it is not a prime number.
 

Professor Greenline from BrightChamps

Using the Prime Factorization Method

We use this method of breaking large numbers into small numbers, then checking the factors. It works only for composite numbers. 


The factors of 21 are 3 and 7. Since there are more than two factors for 21, we can not title 21 a prime number.
 

Max Pointing Out Common Math Mistakes

Common Mistakes to Avoid When Determining if 21 is not Prime Number

Ray Thinking Deeply About Math Problems

FAQs For “Is 21 a Prime Number?

1.Why isn’t 21 a prime number?

Math FAQ Answers Dropdown Arrow

2.Does 21 only have two factors?

Math FAQ Answers Dropdown Arrow

3.Is 21 a twin prime number?

Math FAQ Answers Dropdown Arrow

4.Are 11 and 21 both prime numbers?

Math FAQ Answers Dropdown Arrow

5.What is the next prime number after 21?

Math FAQ Answers Dropdown Arrow

6.How can children in Qatar use numbers in everyday life to understand Is 21 a Prime Number??

Math FAQ Answers Dropdown Arrow

7.What are some fun ways kids in Qatar can practice Is 21 a Prime Number? with numbers?

Math FAQ Answers Dropdown Arrow

8.What role do numbers and Is 21 a Prime Number? play in helping children in Qatar develop problem-solving skills?

Math FAQ Answers Dropdown Arrow

9.How can families in Qatar create number-rich environments to improve Is 21 a Prime Number? skills?

Math FAQ Answers Dropdown Arrow
Professor Greenline from BrightChamps

Important Glossaries for "Is 21 a Prime Number"

  • Composite number- The number which possesses more than 2 factors is named as composite number. Like, example -  10 is factored into 2×5, thus it is a composite number. 

 

  • Factor- A number that divides another number exactly is called a factor. For example, 10/5=2. Here, 5 is a factor.

 

  • Prime Number Chart- A list of prime numbers, often used to quickly determine if a number is prime by checking if it appears in the list.

 

  • Natural number All the numbers that are positive and whole numbers form 1.


 

Professor Greenline from BrightChamps

About BrightChamps in Qatar

At BrightChamps, we know numbers are more than just digits—it’s a way to open doors to countless opportunities! Our mission is to help kids all across Qatar grasp important math skills, like today’s focus on the Is 21 a Prime Number?with a special focus on understanding prime numbers—in a way that’s lively, enjoyable, and easy to follow. Whether your child is figuring out how fast a roller coaster speeds through Qatar’s Angry Birds World, keeping track of scores at a local football match, or managing their allowance to buy the latest gadgets, mastering numbers gives them the confidence they need for everyday challenges. Our interactive lessons make learning both simple and fun. Because kids in Qatar learn in many different ways, we tailor our approach to fit each child’s unique style. From the modern cityscape of Doha to the desert landscapes, BrightChamps brings math to life, making it relatable and exciting throughout Qatar. Let’s make prime numbers a fun part of every child’s math journey!
Math Teacher Background Image
Math Teacher Image

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.

Math Teacher Fun Facts Image
Max, the Girl Character from BrightChamps

Fun Fact

: She loves to read number jokes and games.

INDONESIA - Axa Tower 45th floor, JL prof. Dr Satrio Kav. 18, Kel. Karet Kuningan, Kec. Setiabudi, Kota Adm. Jakarta Selatan, Prov. DKI Jakarta
INDIA - H.No. 8-2-699/1, SyNo. 346, Rd No. 12, Banjara Hills, Hyderabad, Telangana - 500034
SINGAPORE - 60 Paya Lebar Road #05-16, Paya Lebar Square, Singapore (409051)
USA - 251, Little Falls Drive, Wilmington, Delaware 19808
VIETNAM (Office 1) - Hung Vuong Building, 670 Ba Thang Hai, ward 14, district 10, Ho Chi Minh City
VIETNAM (Office 2) - 143 Nguyễn Thị Thập, Khu đô thị Him Lam, Quận 7, Thành phố Hồ Chí Minh 700000, Vietnam
Dubai - BrightChamps, 8W building 5th Floor, DAFZ, Dubai, United Arab Emirates
UK - Ground floor, Redwood House, Brotherswood Court, Almondsbury Business Park, Bristol, BS32 4QW, United Kingdom