15 (number)

For the Punk Band, see Fifteen (band).

15 (fifteen) is the natural number following 14 and preceding 16. In English, it is the smallest integer with seven letters in its spelled out name.


Cardinal	15 fifteen
Ordinal	15th fifteenth
Factorization	$3 \cdot 5$
Divisors	1, 3, 5, 15
Roman numeral	XV
Binary	1111
Hexadecimal	F
Hebrew	ט"ו (Tet Vav)

In mathematics

Fifteen is a triangular number and a hexagonal number, a pentatope number, and the 4th Bell number. Fifteen is the double factorial of 5. It is a composite number; its proper divisors being , and . With only two exceptions, all prime quadruplets enclose a multiple of 15, with 15 itself being enclosed by the quadruplet (11, 13, 17, 19). 15 is also the number of supersingular primes.

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

Fifteen is the magic constant of the unique order-3 normal magic square:

15 is the smallest number that can be factorized using Shor's quantum algorithm.

There are 15 solutions to Znám's problem of length 7.

15 is a repdigit in binary (1111).

In science

The atomic number of phosphorus

Astronomy

Messier object M15, a magnitude 7.5 globular cluster in the constellation Pegasus.
The New General Catalogue object NGC 15, a spiral galaxy in the constellation Pegasus.
The Saros number of the solar eclipse series which began on -2557 July 1 and ended on -1223 September 8. The duration of Saros series 15 was 1334.2 years, and it contained 75 solar eclipses.
The Saros number of the lunar eclipse series which began on -2219 April 30 and ended on -921 June 19. The duration of Saros series 15 was 1298.1 years, and it contained 73 lunar eclipses.

In religion

There were 15 steps between the Ezrat Nashim and the Courtyard of the Temple in Jerusalem, on which the Levites stood and sang during the offering of sacrifices. Fifteen Psalms (Chapters 120-134), collectively known as the "Songs of Ascent", correspond to the 15 steps on which the Levites stood.
The 15th day of any Hebrew month is not written with the letters that represent "10" and "5" (י-ה, yodh and heh), because those are the first two letters of the four-letter name of God. Instead, the date is written with the letters representing "9" and "6" (ט-ו, teth and vav)
Passover begins on the 15th day of the Hebrew month of Nisan
Sukkot begins on the 15th day of the Hebrew month of Tishrei
Tu Bishvat is a Jewish holiday occurring on the 15th of the Hebrew month of Shevat
Shushan Purim (the day on which Purim is celebrated in Jerusalem and a few other cities in Israel) occurs on the 15th day of the Hebrew month of Adar
Tu B'Av is a minor Jewish holiday occurring on the 15th day of the Hebrew month of Av

The age 15

Age 15 is:

The age of a quinceañera, a Hispanic girl celebrating her fifteenth birthday. Thus, Spanish bingo callers might refer to the number 15 as la niña bonita (the beautiful girl).
The age for obtaining a driver's (or learner's) permit in jurisdictions where the age for a driver's license is sixteen.
Under certain circumstances one can be sent to prison to await trial at the age of 15

In other fields

Fifteen is:

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 '15 refers to the first Jacobite Rising in Great Britain in 1715.
The number of checkers each side has in the beginning of a backgammon game.
The 15 puzzle.
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
"Fifteen men on a dead man's chest"
The number of function keys on most Mac keyboards
The name of a modern punk band, Fifteen (band), with lead singer Jeff Ott.
The number of years of marriage celebrated with crystal
The number of letters in the words "uncopyrightable" and "dermatoglyphics", which are the longest words there are without repeating a letter.
One of the "Lost Numbers" on the television show, Lost, along with 4, 8, 16, 23, and 42.
The number of completed, numbered symphonies and string quartets by Dmitri Shostakovich
A film rating and age limit on viewing given by the British Board of Film Classification
The number of The Devil in the major arcana of Tarot cards.
The number to dial for SAMU in case of an emergency in France.
The number of minutes in 1/4 of an hour, 15 min past or before an o'clock is often known as 'Quarter Past' and 'Quarter To' respectivley.

Historical years

15 A.D., 15 B.C., 1915, 2015, etc.

Integers