# 20 (number)

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 ← 19 20 21 →
Cardinal twenty
Ordinal 20th
(twentieth)
Numeral system vigesimal
Factorization 22× 5
Divisors 1, 2, 4, 5, 10, 20
Greek numeral Κ´
Roman numeral XX
Binary 101002
Ternary 2023
Quaternary 1104
Quinary 405
Senary 326
Octal 248
Duodecimal 1812
Hexadecimal 1416
Vigesimal 1020
Base 36 K36

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][2]

## In mathematics

An icosahedron has 20 faces

## In science

### 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".

## 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[7] (e.g. the newspaper headline "Scores of Typhoon Survivors Flown to Manila")[8]

## In sports

A standard dartboard is laid out as 20 sectors

## 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."
2. ^ [1]
3. ^ "Sloane's A000292 : Tetrahedral numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
4. ^ "Sloane's A071395 : Primitive abundant numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
5. ^ "God's Number is 20". Cube20.org
6. ^ Jonathan Fildes (August 11, 2010). "Rubik's Cube quest for speedy solution comes to an end". BBC News.
7. ^ "Biblical Criticism", The Classical Journal 36:71:83ff (March 1827) full text
8. ^ "CBS News", Scores of Typhoon Survivors Flown to Manila (November 2013) [2]
9. ^ "Japan's Age of Majority Changed to 18 - Living the Japon.com". www.japan-experience.com. Retrieved 19 March 2018.