20 (number)

TheInfoList

20 (twenty; Roman numeral XX) is the
natural number In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and total order, ordering (as in "this is the ''third'' largest city in the country"). In common mathematical terminology, w ...
following 19 and preceding 21. A group of twenty units may also be referred to as a score.

In mathematics

*20 is a
pronic number A pronic number is a number which is the product of two consecutive integer An integer (from the Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoke ...
. *20 is a
tetrahedral numberImage:Pyramid of 35 spheres animation.gif, frame, A pyramid with side length 5 contains 35 spheres. Each layer represents one of the first five triangular numbers. A tetrahedral number, or triangular pyramidal number, is a figurate number that repre ...
as 1, 4, 10, 20. *20 is the basis for
vigesimal A vigesimal () or base-20 (base-score) numeral system is based on 20 (number), twenty (in the same way in which the decimal, decimal numeral system is based on 10 (number), ten). ''wikt:vigesimal#English, Vigesimal'' is derived from the Latin adje ...
number systems. *20 is the third
composite number A composite number is a positive integer In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calcul ...
to be the product of a squared prime and a prime, and also the second member of the (''2''2)''q'' family in this form. *20 is the smallest
primitive abundant number In mathematics a primitive abundant number is an abundant number whose proper divisors are all deficient numbers. For example, 20 is a primitive abundant number because: :#The sum of its proper divisors is 1 + 2 + 4 + 5 + 10 = 22, so 20 is an abund ...
. *An
icosahedron In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position ...

has 20 faces. A
dodecahedron In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position o ...

has 20 vertices. *20 can be written as the sum of three
Fibonacci number In mathematics, the Fibonacci numbers, commonly denoted , form a sequence In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), ...

s uniquely, i.e. 20 = 13 + 5 + 2. *The product of the number of
divisors In mathematics, a divisor of an integer n, also called a factor of n, is an integer m that may be multiplied by some integer to produce n. In this case, one also says that n is a multiple of m. An integer n is divisible by another integer m if m ...
and the number of proper divisors of 20 is exactly 20. *20 is the number of moves (quarter or half turns) required to optimally solve a
Rubik's Cube The Rubik's Cube is a 3-D combination puzzle invented in 1974 by Hungarian sculptor and professor of architecture Ernő Rubik. Originally called the Magic Cube, the puzzle was licensed by Rubik to be sold by Ideal Toy Company, Ideal Toy Corp. ...

in the worst case."God's Number is 20"
Cube20.org

In science

*The
atomic number The atomic number or proton number (symbol ''Z'') of a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. ...
of
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

. *The third magic number in physics. *The
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
shower number for
Coma Berenicids Comae Berenicids 'sic''(formerly Coma Berenicids, International Meteor Organization, IMO designation: COM; International Astronomical Union, IAU shower number: 20) is a minor meteor shower with a Radiant (meteor shower), radiant in the constel ...
.

Biology

*The number of
proteinogenic Proteinogenic amino acids are amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known ...
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

s that are encoded by the standard genetic code. *In some countries, the number 20 is used as an index in measuring
visual acuity Visual acuity (VA) commonly refers to the clarity of vision Vision or The Vision may refer to: Perception Optical perception * Visual perception Visual perception is the ability to interpret the surrounding environment (biophysical), ...
. 20/20 indicates normal vision at 20 feet, although it is commonly used to mean "perfect vision". (Note that this applies only to countries using the
Imperial Imperial is that which relates to an empire, emperor, or imperialism. Imperial or The Imperial may also refer to: Places United States * Imperial, California * Imperial, Missouri * Imperial, Nebraska * Imperial, Pennsylvania * Imperial, Texas * ...
system. The metric equivalent is 6/6.) When someone is able to see only after an event how things turned out, that person is often said to have had "20/20 hindsight".

As an indefinite number

* A 'score' is a group of 20 (often used in combination with a cardinal number, i.e. ''fourscore'' to mean 80), but also often used as an indefinite number"Biblical Criticism", ''The Classical Journal'' 36:71:83''ff'' (March 1827
full text
/ref> (e.g. the newspaper headline "Scores of Typhoon Survivors Flown to Manila")."CBS News"
''Scores of Typhoon Survivors Flown to Manila''
(November 2013)

In sports

*
Twenty20 Twenty20 (T20) is a shortened game format of cricket. At the professional level, it was introduced by the England and Wales Cricket Board (ECB) in 2003 for the county cricket, inter-county competition. In a Twenty20 game, the two teams have a ...

is a form of
limited overs cricket Limited overs cricket, also known as one-day cricket, is a version of the sport of cricket Cricket is a bat-and-ball gameBat-and-ball may refer to: *Bat-and-ball games Bat-and-ball games (or safe haven games) are field games played b ...
where each team plays only 20
overs In cricket, an over consists of six consecutive legal delivery (cricket), deliveries bowling (cricket), bowled from one end of a cricket pitch to the player batting at the other end, almost always by a single bowler. A maiden over is an over in ...
. * A standard
dartboard Darts or dart-throwing is a competitive game in which two or more players bare-handedly throw File:Throwing rocks (3843366278).jpg, Throwing of stones into the river Throwing is the launching of a ballistics, ballistic projectile by hand. This ...

is laid out as 20 sectors. * The
Kentucky Derby The Kentucky Derby is a horse racing, horse race held annually in Louisville, Kentucky, United States, almost always on the first Saturday in May, capping the two-week-long Kentucky Derby Festival. The competition is a graded stakes race, Gr ...
currently has a maximum field of 20 horses. * In
rugby union Rugby union, commonly known simply as rugby, is a Contact sport#Terminology, close-contact team sport that originated in England in the first half of the 19th century. One of the Comparison of rugby league and rugby union, two codes of rugby f ...
, 20 national teams currently qualify for each edition of the men's
Rugby World Cup The Rugby World Cup is a men's rugby union Rugby union, commonly known simply as rugby, is a Contact sport#Terminology, close-contact team sport that originated in England in the first half of the 19th century. One of the Comparison of ...
. * In
chess Chess is a board game Board games are tabletop game Tabletop games are game with separate sliding drawer, from 1390–1353 BC, made of glazed faience, dimensions: 5.5 × 7.7 × 21 cm, in the Brooklyn Museum (New Yor ...

, 20 is the number of legal moves for each player in the starting position. * In
Australian rules football Australian rules football, officially known as Australian football, or simply called "Aussie rules", "football Football is a family of team sport A team is a NCAA_men's_basketball_rules,_games_consist_of_20-minute_halves. *_Ice_hockey.html" ;"title="College basketball">NCAA men's basketball rules, games consist of 20-minute halves. * Ice hockey">College basketball">NCAA men's basketball rules, games consist of 20-minute halves. * Ice hockey games are played in three 20-minute periods.

Age 20

*In many disciplines of developmental psychology, adulthood starts at age 20. *Formerly the age of majority in Japan and in Japanese tradition. *Age 20 is the age at which
Levites A Levite (or Levi) (, ) is a Jewish Jews ( he, יְהוּדִים ISO 259-2 , Israeli pronunciation ) or Jewish people are an ethnoreligious group and nation originating from the Israelites Israelite origins and kingdom: "The first act in ...
in the time of King David were allowed "to do the work for the service of the house of the Lord", the
Temple in Jerusalem Two ancient Israelite The Israelites (; he, בני ישראל ''Bnei Yisra'el'') were a confederation of Iron Age ancient Semitic-speaking peoples, Semitic-speaking tribes of the ancient Near East, who inhabited a part of Canaan during th ...
(see
First Chronicles The Book of Chronicles ( he, דִּבְרֵי־הַיָּמִים ) is a Hebrew language, Hebrew prose work constituting part of Judaism, Jewish and Christian scripture. It contains a genealogy starting from Adam, and a narrative of the history ...
Chapter 23, verses 24 and 27). In the time of
Ezra Ezra (; he, עֶזְרָא, '; fl. 480–440 BCE), also called Ezra the Scribe (, ') and Ezra the Priest in the Book of Ezra The Book of Ezra is a book of the Hebrew Bible The Hebrew Bible or Tanakh (; : , or ), is the of scripture ...

and
Nehemiah Nehemiah is the central figure of the Book of Nehemiah, which describes his work in rebuilding Jerusalem during the Second Temple period. He was governor of Yehud Medinata, Persian Judea under Artaxerxes I of Persia (465–424 BC). The name i ...

, following the
Babylonian captivity The Babylonian captivity or Babylonian exile is the period in Jewish history during which a number of people from the ancient Kingdom of Judah were captives in Babylon, the capital of the Neo-Babylonian Empire. After the Battle of Carchemish in ...
, it was
Levites A Levite (or Levi) (, ) is a Jewish Jews ( he, יְהוּדִים ISO 259-2 , Israeli pronunciation ) or Jewish people are an ethnoreligious group and nation originating from the Israelites Israelite origins and kingdom: "The first act in ...
from the age of 20 upwards who were assigned "to oversee the work of the house of the LORD" (
Ezra Ezra (; he, עֶזְרָא, '; fl. 480–440 BCE), also called Ezra the Scribe (, ') and Ezra the Priest in the Book of Ezra The Book of Ezra is a book of the Hebrew Bible The Hebrew Bible or Tanakh (; : , or ), is the of scripture ...
Chapter 3, verse 8).

In other fields

* +20 is the code for international direct dial phone calls to Egypt. *CB slang for a place, being short for the ten-code "10–20" meaning "What is your location?" *''20/20 (US television show), 20/20'' is a primetime newsmagazine program on ABC, which in turn is taken from the expression for normal eyesight (20/20 vision). *''020'' is the ISO 3166-1 numeric 3 digit country code for Andorra. *The UIC Country Code for Russia identifying member countries of the International Union of Railways (UIC). *20 is the value of the letter ''J'' when computing the check digit in the serial number of an intermodal container, intermodal (shipping) container, as defined by ISO 6346. *In Hebrew numerals, the letter Kaph (כ) represents twenty; see also gematria, the Hebrew system of numerology. Similarly the Arabic alphabet, Arabic letter kāf (ك) represents twenty in the Abjad numerals. *The number of questions in the game Twenty Questions.