Two numbers are relatively prime when their greatest common divisor is 1.

Example:

The factors of 15 are 1, 3, 5, and 15.

The factors of 8 are 1, 2, 4, and 8.

The greatest common divisor is the largest factor two numbers have in common.

The largest number that appears in both sets of factors is 1, so the greatest common divisor is 1.

Hence, 8 and 15 are relatively prime.