Question

what is co- prime numbers

Answer 1

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.

Answer 2

In number theory, two integers a and b are said to be relatively prime, mutually prime, or coprime (also written co-prime) if the only positive integer (factor) that divides both of them is 1. ... This is equivalent to their greatest common divisor being 1.