15

Fifteen is the natural number that follows fourteen and precedes sixteen. It is an odd, composite number with prime factors 3 and 5.

In decimal notation it is written as 15; in other bases, 15 is 1111 in binary, 17 in octal, and F in hexadecimal. The Roman numeral for fifteen is XV.

The divisors of 15 are 1, 3, 5, and 15. It is a semiprime (the product of two distinct primes) and the fifth triangular number, since 1 + 2 + 3 + 4 + 5 equals 15.

The sum of its divisors is 24, so the sum of proper divisors is 9; hence 15 is a deficient number.

In science, atomic number 15 corresponds to phosphorus, a nonmetal essential for living organisms and a component of DNA, RNA, and cell membranes. Phosphorus occurs naturally in phosphate minerals and is important in agriculture as a fertilizer.

In culture, the number is associated with the quinceañera, a traditional celebration of a girl’s 15th birthday in many Latin American communities. The 15 puzzle, a famous sliding-puzzle game, uses a 4x4 grid and fifteen numbered tiles.

In everyday use, 15 minutes comprise a quarter of an hour, and the 15th letter of the Latin alphabet is O.