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]