15 (number)

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

In spoken English, the numbers 15 and 50 are often confused because they sound similar. When carefully enunciated, they differ in which syllable is stressed: 15 vs 50 . However, in dates such as 1500 ("fifteen hundred") or when contrasting numbers in the teens, the stress generally shifts to the first syllable: 15 .

Video 15 (number)

In mathematics

15 is:

• a triangular number.
• a hexagonal number.
• a pentatope number.
• the 5th Bell number (i.e. the number of partitions for a set of size 4).
• a composite number; its proper divisors being 1, 3 and 5.
• the number of supersingular primes.
• a repdigit in binary (1111) and quaternary (33). In hexadecimal, as well as all higher bases, 15 is represented as F.
• the smallest number that can be factorized using Shor's quantum algorithm.
• the magic constant of the unique order-3 normal magic square:

There are 15 perfect matchings of the complete graph K₆ and 15 rooted binary trees with four labeled leaves, both of these being among the types of object counted by double factorials. With only two exceptions, all prime quadruplets enclose a multiple of 15, with 15 itself being enclosed by the quadruplet (11, 13, 17, 19).

Because 15 is the product of distinct Fermat primes, 3 and 5, a regular polygon with 15 sides is constructible with compass and unmarked straightedge, and cos ?/15 is expressible in terms of square roots (see here).

If a positive definite quadratic form with integer matrix represents all positive integers up to 15, then it represents all positive integers via the 15 and 290 theorems.

Maps 15 (number)

In science

• The atomic number of phosphorus.
• Group 15 of the periodic table are sometimes known as the pnictogens.

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In Pakistan

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.

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In Sunnism

The Hanbali Sunni madhab states that the age of fifteen of a solar or lunar calendar is when one's taklif (obligation or responsibility) begins and is the stage whereby one has his deeds recorded.

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In Judaism

• In the Hebrew numbering system, the number 15 is not written according to the usual method, with the letters that represent "10" and "5" (?-?, 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" (?-?, 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

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In sports

• In all four of the major Gaelic games--hurling, Gaelic football for men and women, and camogie--each team has 15 players on the field at any given time.
• In rugby union:
  • Each team has 15 players on the field at any given time.
  • The jersey number 15 is worn by the starting fullback.
• In tennis, the number 15 represents the first point gained in a game.

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Age 15

Age 15 is:

• The age of a quinceañera, a Hispanic girl celebrating her 15th 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 legal age for a driver's license is 16.
• In the UK a minor can be sent to prison to await trial at the age of 15.
• In some 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.

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In Music

• "Fifteen" was a song composed by Taylor Swift in her album Fearless in 2008.

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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, Marine Corps, and Air Force Lieutenant Generals, and Navy and Coast Guard Vice Admirals.
• The number of checkers each side has at the start of a backgammon game.
• The 15 puzzle.
• The number of balls in the eight ball variant of billiards
• 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.
• There are 15 Luo Points.

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References

• Wells, D. The Penguin Dictionary of Curious and Interesting Numbers London: Penguin Group. (1987): 91-93

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External links

• Clewett, James. "15: f in hexadecimal". Numberphile. Brady Haran.  - discussing hexadecimals
• Bowley, Roger. "15: Bumfit". Numberphile. Brady Haran.  - discussing the Celtic number as used in Lincolnshire

Source of the article : Wikipedia