20 (number)
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20 Number
 ← 19 20 21 →
Cardinaltwenty
Ordinal20th
(twentieth)
Numeral systemvigesimal
Factorization22 × 5
Divisors1, 2, 4, 5, 10, 20
Greek numeral
Roman numeralXX
Binary101002
Ternary2023
Quaternary1104
Quinary405
Senary326
Octal248
Duodecimal1812
Vigesimal1020
Base 36K36

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 .
4. ^ "Sloane's A071395 : Primitive abundant numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved .
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 2018.