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← 19 21 → 20
← 20 21 22 23 24 25 26 27 28 29 → List of numbers — Integers 0 10 20 30 40 50 60 70 80 90 →
Cardinal	twenty
Ordinal	20th (twentieth)
Numeral system	vigesimal
Factorization
Divisors	1, 2, 4, 5, 10, 20
Roman numeral	XX
Binary	101002
Octal	248
Duodecimal	1812
Hexadecimal	1416

"Twenty" redirects here. For the village in England, see Twenty, Lincolnshire.

20 (twenty) is the natural number following 19 and preceding 21. A group of twenty units may also be referred to as a score.^[1]

In mathematics

Twenty is the third composite number comprising the product of a squared prime and a prime. It is the second composite number of the form p²q; a square-prime, and also the second member of the (2²)q family in this form. 20 has an aliquot sum of 22 (110% in abundance). Accordingly, 20 is the third abundant number and demonstrates an 8 member aliquot sequence; {20,22,14,10,8,7,1,0} 20 is the 4th composite number in the 7-aliquot tree. Two numbers have 20 as their aliquot sum; the discrete semiprime 34 and the squared prime 361. Only 2 other square primes are abundant 12 and 18.

• An icosahedron has 20 faces. A dodecahedron has 20 vertices.
• 20 can be written as the sum of three Fibonacci Numbers uniquely, i.e. 20 = 13 + 5 + 2.
• The product of the number of divisors and the number of proper divisors of 20 is exactly 20.

An icosahedron has 20 faces

In science

• The atomic number of calcium.
• The third Magic number (physics).

Biology

• The number of proteinogenic amino acids that are encoded by the standard genetic code.
• In some countries, the number 20 is used as an index in measuring visual acuity. 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 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".
• There are 20 baby teeth in the deciduous dentition.

In religion

• Age 20 is the age at which Levites 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 (see First Chronicles Chapter 24, verses 24 and 27). In the time of Ezra and Nehemiah, following the Babylonian captivity, it was Levites from the age of 20 upwards who were assigned "to oversee the work of the house of the LORD" (Ezra Chapter 3, verse 8).

In sports

A standard dartboard has 20 segments

• 20 is the retired number of Pro Football Hall of Fame running back Barry Sanders, formerly of the Detroit Lions.
• The retired number of former baseball players Frank Robinson, Mike Schmidt, Mark Fidrych and Lou Brock.
• 20 was the cup number of two-time Nascar Sprint Cup Series champion Tony Stewart. After Stewart left the #20 team in 2009 to drive for himself, Rookie Joey Logano took over the car.
• The Twenty20 Cup is a form of limited Overs Cricket where each team plays only 20 Overs.
• The number of NFL players Ed Reed, Ronde Barber and Brian Dawkins.
• 20 is the number worn by NBA player Manu Ginóbili of the San Antonio Spurs.
• 20 is the number worn by NBA point guard Gary Payton formerly of Seattle SuperSonics
• 20 is the iconic number worn by Soccer player Ole Gunnar Solskjær of Manchester United.
• 20 is the number worn by New York Yankees all-star catcher Jorge Posada.
• A twenty-game winner (pitcher) in baseball is acclaimed as having reached a milestone in a season.
• Lukas Podolski, German National soccer player, wears number 20.
• Polish soccer player Zbigniew Boniek wore number 20 in the National Team.
• 20 is worn by NHL all-star goaltender Evgeni Nabokov of the San Jose Sharks.
• 20 is worn by the MLS player Taylor Twellman of the New England Revolution.
• 20 is worn by NBA player Ray Allen, who currently plays for the Boston Celtics.
• 20 is worn by Portugal's midfielder Deco.
• 20 is worn by NFL player Thomas Jones of the New York Jets.
• 20 is worn by MLB player Carlos Quentin of the Chicago White Sox.
• 20 is the number of Rugby League premierships won by the South Sydney Rabbitohs.
• 20 is worn by Michael Dawson of Premier League team Tottenham Hotspur.
• 20 was worn by Ole Gunnar Solskjaer of Premier League team Manchester United.
• 20 was worn by NHL Hall of Famer Luc Robitaille and his jersey number has been retired by the Los Angeles Kings.

Age 20

• Twenty is the age of majority in Japanese tradition. Someone who is exactly twenty years old is described as hatachi
• The age to be an adult in some cultures.

In other fields

• The number of twenty can also be called a score (as Abraham Lincoln did in his Gettysburg Address).
• Twenty questions is a popular party game
• The Twenty Year Curse refers to the pattern of presidents of the United States who were elected to office in 1840, 1860, 1880, 1900, 1920, 1940, and 1960 to die in office. This pattern ended with the 1980 presidency of Reagan, who survived his time in office and, notably, an attempted assassination.
• Bands with the number twenty in their name include Matchbox Twenty
• In the 1974 sci-fi film Dark Star, Exponential Thermostellar Bomb number 20 threatens to detonate in the Dark Star's bomb bay
• A 20-minute-long program of advertisements and trailers shown before some films playing in American movie theaters is called "The Twenty" (spelled "The 20wenty")
• Yan Tan Tethera is a 20-word jingle for counting sheep
• The ordinal adjective is vicenary
• 20/20 is a late-night newsmagazine program on the ABC network, that has been hosted by Barbara Walters, Hugh Downs, Elizabeth Vargas, and others
• In the roleplaying game Dungeons and Dragons (as well as other RPGs that use twenty-sided dice), twenty-sided dice play a pivotal role in gameplay, and to "roll a twenty" is significant to the point that it is sometimes used in other, usually related, contexts, similar to the use of "doubles" in reference to Monopoly.
• Cigarettes are usually packaged with 20 in each pack.
• "T" is the 20th letter in the alphabet.
• According to The Strokes in the song You Only Live Once there are 20 ways to see the world, and 20 ways to start a fight.

20 is:

• Twenty, a village in Lincolnshire
• In the United States Constitution, $20 is the threshold value of civil disputes above which the right to trial by jury is preserved
• A denomination of U.S. dollar featuring Andrew Jackson's portrait
• A denomination of Pound sterling featuring Adam Smith's portrait
• The code for international direct dial phone calls to Egypt
• The designation of Interstate 20, a freeway that runs from Texas to South Carolina
• 20 (album), a 1988 album by Harry Connick, Jr.
• Twenty (album), a 1997 album by Lynyrd Skynyrd
• 20 (Terminaator album), a 2007 album by Terminaator
• Twenty (concert), a 2006 concert celebrating the 20th Anniversary of Regine Velasquez held at the Araneta Coliseum, Philippines. It was awarded Best Major Concert Act by the Aliw Awards.
• CB slang for "a place", being short for "10-20", used in reference to a person or object's location
• One of the TCP/IP "well-known ports", port 20 being used for File Transfer Protocol
• In the French and Portuguese education system, grades are given out of 20. Giving 20/20 is a very rare occasion.

Historical years

20 A.D., 20 B.C., 1920, 2020, etc.

References

1. ^ John H. Conway and Richard K. Guy, The Book of Numbers. New York: Copernicus (1996): 11. ""Score" is related to "share" and comes from the Old Norse "skor" meaning a "notch" or "tally" on a stick used for counting. ... Often people counted in 20s; every 20th notch was larger, and so "score" also came to mean 20."