# Number System : Co-Primes or Relatively Prime

**Co-Prime or Relatively Prime…Hmm**So what do we mean by Co-Primes or Relatively Prime Nos. ?

A set of numbers which do not have any other common factor other than 1 are called co-prime or relatively prime numbers.

**This means those numbers whose HCF is 1.**

For e.g. : 14 and 15 have no other common factor other than 1 so they are co-prime numbers.

Also called **strangers.****Properties of Co-prime Numbers:**

~ All prime numbers are co-prime to each other.

~Any 2 consecutive integers are always co-prime.

~ Sum of any two co-prime numbers is always co-prime with their product.

~1 is co-prime with all numbers.

~ a and b (natural numbers) are co-prime only if the numbers 2a-1 and 2b-1 are co-prime.

~ Decimals of Co-Prime Nos. never match

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If two numbers a and b are co-prime and the natural number p is divisible by both a and b separately, then the number p is also divisible by a x b.**Decimals of Co-Prime Nos. never match**

So when a number P is divisible by 14 and Q is visible by 15, what are the chances of result being an integer?

Ans. 0 , as 14 and 15 are co-primes.

**The addition or difference of two numbers whose denominators are co-prime will never result in an integer**

P/14 + Q/15 –> Not an integer

P/14 – Q/15 –> Not an integer

The addition or difference of two numbers whose denominators are** not co-prime** may result in an integer.

P/3 + Q/9 — > May result in an integer.

For e.g. 10/3 +24/9

Here 10 is not divisible by 3 and 24 is not divisible by 9. But decimal values are such that on adding it becomes an integer.

Q. Find all the five-digit numbers of the form **46X3Y** that are divisible by 36. ?

This is an interesting question based on the concept of co-prime numbers. Since 36 is a product of 2 co-prime numbers 4 & 9. So if **46X3Y **is divisible by 4 & 9, it will also be divisible by 36.

Lets check the divisibilty of **46X3Y** by 4. To be divisible by 4 the last 2 digits of the number must be divisible by 4. Hence Y can be 2 or 6.

If Y is 2, No. becomes 46X32. Sum of digits = 15+X . For it to be divisible by 9, X= 3**So No is 46332.**

If Y is 6, No. Becomes 46X36. Sum of digits = 19+X. For it to be divisible by 9, X = 8.**So No. is 46836.**

Solve questions from : **Quantitative Aptitude for CAT**