# 20 (number)

20 (twenty) is the natural number following 19 and preceding 21.

 Template:Numbers 20s Cardinal twenty Ordinal twentieth Numeral system vigesimal Factorization ${\displaystyle 2^{2}\cdot 5}$ Roman numeral XX Binary 10100 Hexadecimal 14

## In mathematics

Twenty is a composite number, its proper divisors being 1, 2, 4, 5 and 10. It is also the sum of the first four triangular numbers, making it a tetrahedral number. It is a Harshad number and a self number.

It 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 old UK 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 science

In astronomy,

Messier object M20, a magnitude 5.0 nebula/open cluster in the constellation Sagittarius, also known as the Trifid Nebula.
The New General Catalogue object NGC 20, a spiral galaxy in the constellation Andromeda
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.

## In other fields

In a game of chess both players have 20 first moves from which to choose.

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". 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".

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 the president of the United States elected every twenty years starting with 1840 to die in office. This pattern failed when Reagan survived his time in office.

Bands with the number twenty in their name include Matchbox 20.

A group of twenty units is often referred to as a score.

A curious survival from the Celtic languages is preserved in folklore from the north of England; this is called the "Lincolnshire Score" or the Shepherd's Score. This is a tally that counts twenty units using the rhyming jingle:

Yan, tan, tethera, pethera, pimp,
sethera, lethera, hovera, dovera, dick,