20 (number)

20 (twenty) is the natural number following 19 and preceding 21. A group of twenty units is often referred to as a score.

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[edit] In mathematics

Twenty is a composite number, its proper divisors being 1, 2, 4, 5 and 10. Skipping 2 and adding the other divisors gives 20, hence 20 is a semiperfect number, and since it is not divisible by any of the smaller semiperfect numbers, it is a primitive semiperfect number. 20 is the smallest abundant number that is not a multiple of 3.

It is also the sum of the first four triangular numbers, making it a tetrahedral number.

20 is the total number of digits on the human body, which makes it a possible candidate for a numeral system.

Twenty is used as a radix for vigesimal number systems. In the former British currency system, there were twenty shillings in a pound.

A polyhedron with twenty faces is an icosahedron.

Twenty is the first number in an early Indian magic square of order 3.

In base 10, it is a Harshad number and the smallest even self number.

The Saros number of the solar eclipse series which began on -2286 June 5 and ended on -1006 July 13. The duration of Saros series 20 was 1280.1 years, and it contained 72 solar eclipses.

The Saros number of the lunar eclipse series which began on -1966 March 25 and ended on -668 May 12. The duration of Saros series 20 was 1298.1 years, and it contained 73 lunar eclipses.

The number of proteinogenic amino acids that are encoded by the standard genetic code.
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".

In Judaism, 20 is the age at which a person is responsible for his actions to the Heavenly Court (and is thus culpable for his sins)

The retired number of former baseball stars Frank Robinson and Mike Schmidt.
20 is the number of Tony Stewart's 2-time winning NASCAR NEXTEL Cup Series car.
The Twenty20 Cup is a form of limited Overs Cricket where each team plays only 20 Overs.

20 chairs on Sesame Street.

The number of twenty can also be called a score.
In a game of chess both players have 20 first moves from which to choose
Twenty questions is a popular party game
Twenty is the age of majority in Japanese tradition. Someone who is exactly twenty years old is described as hatachi
The "Twenty Year Curse" refers to the pattern of presidents of the United States who assumed 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
Bands with the number twenty in their name include matchbox twenty and 20goto10.
A 20-minute-long entertainment program shown before some films playing in United Artists 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

20 is:

The code for international direct dial phone calls to Egypt
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 with Andrew Jackson's portrait
A denomination of Pound sterling with Sir Edward Elgar's portrait
The designation of Interstate 20, a freeway that runs from Texas to South Carolina
Twenty (album), a 1997 album by Lynyrd Skynyrd
CB radio jargon for "a place", being short for "10-20", used in reference to a person or object's location
One of the TCP/IP ports used for File Transfer Protocol