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15 (fifteen) is the natural number following 14 and preceding 16.

1 In mathematics

Fifteen is a triangular number and a hexagonal number, and the 4th Bell number. Fifteen is the double factorial of 5. It is a composite numberA composite number is a positive integer with three or more divisors. Every integer (excluding one and zero) is either a prime number or a composite number. The integer 14 is a composite number because it has divisors 1, 2, 7, and 14 (four divisors).; its proper divisors being 1, 3 and 5. With only two exceptions, all prime quadrupletA prime quadruplet is a group of four primes, consisting of two pairs of twin primes separated only by three non-primes, specifically, a multiple of 2, a multiple of 15 and another multiple of 2. From the smallest prime p of the quadruplet, the other prims enclose a multiple of 15, with 15 itself being enclosed by the quadruplet (11, 13, 17, 19).

In hexadecimal, as well as all higher bases, fifteen is represented as F.

Fifteen is the magic constantThe magic constant of a magic square, an n by n matrix, is defined such that the sum of any row, column or main diagonal yields the same result, denoted M ''n . If the numbers in the magic square are 1, 2,. n , then. Paul Muljadi discovered and proved the of a 3 by 3 magic square:

2 In other fields

• The Lo Shu Square, used in Feng Shui, is a 3x3 magic square with magic constant 15.

Age 15 is:

• The age of a quinceañera, a Hispanic girl celebrating her fifteenth birthday.
• The age for obtaining a driver's (or learner's) permit in jurisdictions where the age for a driver's license is sixteen.

Fifteen is:

• The atomic number of phosphorus
• The number of days in each of the 24 cycles of the Chinese calendar.
• The number of guns in a gun salute to Army, Air Force and Marine Corps Lieutenant Generals, and Navy and Coast Guard Vice Admirals.
• The designation of Interstate 15, a freeway that runs from California to Montana.
• The number of players on a rugby union team.
  • In rugby union the full back wears the 15 shirt.
• The number of checkers each side has in the beginning of a backgammon game.
• Fifteen, a television program.
• A restaurant in London, built by celebrity chef Jamie Oliver
• In the United States, the customary percentage to tip a waitress in a restaurant by one or two guests
• The number of balls in the eight ball variant of billiards
• The number of function keys on most Mac keyboards
• Historical years: 15 A.D., 15 B.C., or 1915