- 15 (number)
15 Cardinal fifteen Ordinal 15th
Factorization Divisors 1, 3, 5, 15 Roman numeral XV Binary 11112 Octal 178 Duodecimal 1312 Hexadecimal F16 Hebrew ט"ו (Tet Vav)
In speech, the numbers 15 and 50 are often confused. When carefully enunciated, they differ in which syllable is stressed: 15 /fɪfˈtiːn/ vs 50 /ˈfɪfti/. However, in dates such as 1500 ("fifteen hundred") or when contrasting numbers in the teens, the stress generally shifts to the first syllable: 15 /ˈfɪftiːn/.
Fifteen is a triangular number, 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 1, 3 and 5. 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.
15 is the 4th discrete semiprime (3.5) and the first member of the (3.q) discrete semiprime family. It is thus the first odd discrete semiprime. The number proceeding 15; 14 is itself a discrete semiprime and this is the first such pair of discrete semiprimes. The next example is the pair commencing 21.
The aliquot sum of 15 is 9, a square prime 15 has an aliquot sequence of 6 members (15,9,4,3,1,0). 15 is the fourth composite number in the 3-aliquot tree. The abundant 12 is also a member of this tree. Fifteen is the aliquot sum of the consecutive 4-power 16, and the discrete semiprime 33.
15 and 16 form a Ruth-Aaron pair under the second definition in which repeated prime factors are counted as often as they occur.
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.
- The atomic number of phosphorus.
- Group 15 of the periodic table are sometimes known as the "Pnictogens".
15 Madadgar is designated as an emergency number in Pakistan, for mobile phones, similar to the international GSM emergency number 112, if 112 is used in Pakistan, then the call is routed to 15. 112 can be used in an emergency even if the phone is locked and does not have a sim-card in it.
- In the Hebrew numbering system, the number 15 is not written according to the usual method, with the letters that represent "10" and "5" (י-ה, yodh and heh), because those spell out one of the Jewish names 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
- In tennis, the number 15 represents the first point gained in a game.
- In Rugby Union the full back wears the number 15, it is also the number of players on the field.
- In Baseball, New York Yankees Catcher, Thurman Munson wore number 15, it was eventually retired by the Yankees organization.
- In U.S. college athletics, schools that are members of NCAA Division I are allowed to provide athletic scholarships to a maximum of 15 women's basketball players in a given season.
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 certain jurisdictions, including some where the age for a driver's license is sixteen.
- In the UK a minor can be sent to prison to await trial at the age of 15.
- In many countries it is the legal age for sexual intercourse. 
- In the UK it is the legal age to watch a BBFC "15" rated film in a cinema or purchase a "15" rated film or video game.
- The number of days in each of the 24 cycles of the Chinese calendar.
- The number of guns in a gun salute to Army, Marine Corps, and Air Force Lieutenant Generals, and Navy and Coast Guard Vice Admirals.
- The designation of Interstate 15, a freeway that runs from California to Montana.
- The designation of U.S. Route 15, a highway that runs from South Carolina to New York.
- The number of checkers each side has at the start of a backgammon game.
- The 15 puzzle.
- A restaurant in London, built by celebrity chef Jamie Oliver
- 15 minutes of fame
- The 15th is a Wire song
- The number of balls in the eight ball variant of billiards
- The number of letters in the words "uncopyrightable", "dermatoglyphics", "misconjugatedly", and "hydropneumatics", which are the longest words in the English language that do not repeat a letter.
- 15 is one of The Numbers – 4, 8, 15, 16, 23, and 42 – featured in Lost.
- The number to dial for SAMU in case of an emergency in France.
- The number of minutes in one quarter of an hour; 15 minutes past or before an hour is often known as quarter past and quarter to, respectively.
- The number of the French department Cantal
- The number corresponding to The Devil in tarot cards.
- A Taylor Swift song.
15 A.D., 15 B.C., 1715, 1815, 1915...
- Wells, D. The Penguin Dictionary of Curious and Interesting Numbers London: Penguin Group. (1987): 91–93
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