Definitions We know that a factor of n divides n exactly. A multiple of n is n, 2n, 3n, …A prime has exactly two distinct positive factors: 1 and itself. A composite has more than two positive factors. 1 is neither prime nor composite. Facts to keep 2 is the only even prime. 2, 3, 5, 7, 11, 13, 17, 19, 23 are the primes less than 25. 9 = 3 × 3 is composite. 15 = 3 × 5 is composite.