What is a co-prime number?
Numbers, which do not have any common factor between them other than one, are called co-prime numbers.
When two numbers (they may not be prime) do not have any common factor other than one between them they are called co-prime or relatively prime.
It is obvious that two prime numbers are always co-prime. For example,
However, even two composite numbers can also be co-prime or relatively prime. For example