Two numbers are "relatively prime" when they have no common factors other than 1.
In other words you cannot evenly divide both by some common value.
Examples: • 7 and 20 are relatively prime (no common factor) • 6 and 20 are not relatively prime because you can evenly divide both by 2 (2 is a common factor)
reference:
source: https://www.mathsisfun.com/definitions/relatively-prime.html [accessed on 18/03/2015]