Co-Prime Numbers

What are Co-Prime Numbers?

Co-prime number is a set of numbers or integers which have only 1 as their common factor i.e. their highest common factor (HCF) will be 1. Co-prime numbers are also known as relatively prime or mutually prime numbers. It is important that there should be two numbers in order to form co-primes.

Co-Prime Numbers

How to Find Co-prime Numbers?

Consider a set of two numbers, if they have no positive integer that can divide both, other than 1, the pair of numbers is co-prime.

Example 1: 21 and 22

For 21 and 22:

  • The factors of 21 are 1, 3, 7 and 21.
  • The factors of 22 are 1, 2, 11 and 22.

Here 21 and 22 have only one common factor that is 1. Hence, their HCF is 1 and are co-prime.

Example 2: 21 and 27

For 21 and 27:

  • The factors of 21 are 1, 3, 7 and 21.
  • The factors of 27 are 1, 3, 9 and 27.

Here 21 and 27 have two common factors they are 1 and 3. HCF is 3 and they are not co-prime.

Properties of Co-Prime Numbers

Some of the properties of co-prime numbers are as follows.

  • 1 is co-prime with every number.
  • Every prime number is co-prime to each other: As every prime number have only two factors 1 and the number itself, the only common factor of two prime numbers will be 1. For example, 2 and 3 are two prime numbers. Factors of 2 are 1, 2 and factors of 3 are 1, 3. The only common factor is 1 and hence is co-prime.
  • Any two successive numbers/ integers are always co-prime: Take any consecutive number such as 2, 3 or 3, 4 or 5, 6 and so on; they have 1 as their HCF.
  • The sum of any two co-prime numbers are always co-prime with their product: 2 and 3 are co-prime and have 5 as their sum (2+3) and 6 as the product (2×3). Hence, 5 and 6 are co-prime to each other.

Example Question Related to Co-prime Numbers

Question: Check whether 13 and 31 are co-prime.


13 and 31 are two prime numbers; therefore, they are co-prime to each other. (Property 2)

The factors of 13 are 1, 13 and the factors of 31 are 1, 31.

They have only 1 as their common factor. So, they are coprime numbers.

Frequently Asked Questions From Co-Prime Numbers

What are Co-prime Numbers?

Co-prime numbers or relatively prime numbers are those numbers which have their HCF (Highest Common Factor) as 1. In other words, two numbers are co-prime if they no common factor other than 1.

What is the difference between prime and Coprime numbers?

A prime number is defined as a number which has no factor other than 1 and itself. On the contrary, coprimes are considered in pairs and two numbers are co-prime if they have no common factors other than 1.

How do you Find the Co-prime of a Number?

To find the co-prime of a number, find the HCF of the number first. Then, choose any number and take the HCF. All the numbers which do not have any common factor other than will be the co-prime of the chosen number.

Is 18 and 35 Coprime Numbers?

Yes, 18 and 35 are co-prime numbers. The factors of 18 are 1, 2, 3, 6, 9 and 18 while the factors of 35 are 1, 5, 7 and 35. Since the HCF is 1, they are coprime.

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