 # Co-Prime Numbers

## What are the Co-Prime Numbers?

A 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. ## 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.

Test your Knowledge on Co – Prime Numbers!

## Properties of Co-Prime Numbers

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

• 1 is co-prime with every number.
• Any two prime numbers are co-prime to each other: As every prime number has 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 they are co-prime.
• Any two successive numbers/ integers are always co-prime: Take any consecutive numbers 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.
• Two even numbers can never form a coprime pair as all the even numbers have a common factor as 2.
• If two numbers have their unit digits as 0 and 5, then they are not coprime to each other. For example 10 and 15 are not coprime since their HCF is 5 (or divisible by 5).

### Co prime numbers from 1 to 100

There are several pairs of co-primes from 1 to 100 which follow the above properties. Some of them are:
(13, 14)
(28, 57)
(1, 99)
(2, 97)
(46, 67)
(75, 41) and so on.
Also, we can write any number with the combination of 1 as a coprime pair such as (22, 1), (31, 1), (4, 1), (90, 1), (1, 100). In this way, many coprime numbers are defined from 1 to 100.

### Example Question Related to Co-prime Numbers

Question 1: Check whether 13 and 31 are co-prime.

Solution:

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.

Question 2: Check whether 150 and 295 are coprimes.

Solution:

Given two number are: 150 and 295
150 and 295 are divisible by 5.
From the properties of coprime numbers, 150 and 295 are not coprime.
Alternatively,
150 = 2 × 3 × 5 × 5
295 = 5 × 59
HCF(150, 295) = 5 ≠ 1
Therefore, 150 and 295 are not coprime.

## Frequently Asked Questions From Co-Prime Numbers

### What are Co-prime Numbers?

Co-prime numbers or relatively prime numbers are those numbers that 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 that has no factor other than 1 and itself. On the contrary, co-primes 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 factors of the number first. Then, choose any number and find the factors of the chosen number. All the numbers which do not have any common factor other than 1 will be the co-prime of the given number.

### Are 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.

### Is 1 Coprime to All numbers?

Yes, 1 is coprime to all the numbers. Since HCF of 1 and any number is 1 itself. Hence, by the definition of coprime numbers, 1 said to be coprime with all numbers.

1. Ravi

Sir,
I think that last property of co prime number is not right. For example 5 and 6 is co prime number but their sum 11 and their product 30 is not co prime number.

If I am wrong then please tell me.

Thank you

1. lavanya

11 and 30 are coprime numbers because they both are commonly divisible only by 1 and no other factor.
11 = 1 x 11
30 = 1 x 2 x 3 x 5
As, we can see here, there is no common factor other than 1 between 11 and 30.

2. Manpreet

Which number are coprime
a 18,35

1. lavanya

18 and 35 are coprime numbers because they both are commonly divisible only by 1 and no other factor.
18 = 1 x 2 x 3 x 3
35 = 1 x 5 x 7
Hence, we can see there is no common factors between 18 and 35 other than 1.

3. Ananya Pandey

Mam/Sir,
I have a doubt in two questions that’s was:-

1. Write the smallest digit and the greatest digit in the blank space of each of the following numbers so that the number formed is divisible by 3:
(a) _6724
(b) 4765_2

2. Write a digit in the blank space of each of the following numbers so that number formed is divisible by 11:
(a) 92_389
(b) 8_9484

Thank you

1. vijay

a) ans is 2 add the all the numbers including 2 sum is 21 which is divisible by 3 hence the whole number 26724 is divisible by 3

4. Anaya

Is a number co prime with itself?
Like is 10 co prime with 10?

5. REYA SHIWANI

6. Sinan

7. Which of the following pair is co-prime *

7. Doodh Nath Tiwari

is 1 and 1 a coprime no.

8. Shalini

9. Lucky Pandey

What is the difference between co prime number and relatively prime number?

1. lavanya

Relatively prime numbers are also called coprime numbers. These numbers do not have any common factors other than 1.
For example, 21 and 22 are relatively prime or coprime.

10. Ansh vishnoi

Ok sir
Sir what is the real number