Numbers / 15

The Number 15

15 is composite: factorization 3 × 5. That gives 15 exactly 4 divisors: 1, 3, 5, 15. 15's proper divisors total just 9, 6 short of 15, the deficient case most of 15's neighbors share too. 15 is also the 5th triangular number, the sum of 1 through 5. 15 has a digit sum of 6; for 15, the digital root works out to 6. 15: 3, 5 all divide 15 evenly, out of 2-12. 15 is 1111 in binary and F in hex; as a Roman numeral it's XV.