Two or more natural numbers are relatively prime if they have a greatest common factor (gcf) of 1.

Another way to state that a set of numbers are relatively prime is to say that they are coprime.

Consider the numbers 20 and 63.

20 = 2²5¹

63 = 3²7¹

gcf = 1

Since the greatest common factor of 20 and 63 is 1, 20 and 63 are relatively prime.

Consider the numbers 20 and 50.

40 = 2³5¹

50 = 2¹5²

gcf = 2¹5¹

gcf = 10

Since the greatest common factor of 40 and 50 is 10 (and NOT 1), 40 and 50 are NOT relatively prime.