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

In Mathematics

Twenty is a composite number. It is also the smallest primitive abundant number. The Happy Family of sporadic groups is made up of twenty finite simple groups that are all subquotients of the friendly giant, the largest of twenty-six sporadic groups.

Geometry

An icosagon is a polygon with 20 edges. Bring's curve is a Riemann surface, whose fundamental polygon is a regular hyperbolic icosagon.

Platonic Solids

The largest number of faces a Platonic solid can have is twenty faces, which make up a regular icosahedron. A dodecahedron, on the other hand, has twenty vertices, likewise the most a regular polyhedron can have. This is because the icosahedron and dodecahdron are duals of each other.

Other fields

Science

20 is the third magic number in physics.

Biology

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" in 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"

Psychology

In many disciplines of developmental psychology, adulthood starts at age 20.

Culture

Age 20

The traditional age of majority in Japan, although the voting age has been reduced to 18. Japanese people commemorate the twentieth birthday with personal ceremonies, and it comes with a number of legal rights like the right to marry. To represent this, the Japanese language has a special word for "20-years-old" that does not follow the rest of their numbering system. Accordingly, the word 二十歳 is read all at once as "はたち" (hatachi) rather than the expected pronunciation of the three characters as "にじゅうさい" (nijyuusai, which is literally "two," "ten," and the counter for "years old").

Number systems

20 is the basis for vigesimal number systems, used by several different civilizations in the past (and to this day), including the Maya.

Indefinite number

A 'score' is a group of twenty (often used in combination with a cardinal number, e.g. fourscore to mean 80), but also often used as an indefinite number (e.g. the newspaper headline "Scores of Typhoon Survivors Flown to Manila").

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, so "score" also came to mean 20."
2. "score | Origin and meaning of score by Online Etymology Dictionary". www.etymonline.com. Retrieved 2020-08-16.
3. "Sloane's A071395 : Primitive abundant numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
4. Weber, Matthias (2005). "Kepler's small stellated dodecahedron as a Riemann surface" (PDF). Pacific Journal of Mathematics. 220 (1): 172. doi:10.2140/pjm.2005.220.167. MR 2195068. S2CID 54518859. Zbl 1100.30036.
5. Weisstein, Eric W. "Icosahedron". mathworld.wolfram.com. Retrieved 2020-08-16.
6. Weisstein, Eric W. "Dodecahedron". mathworld.wolfram.com. Retrieved 2020-08-16.
7. "Definition of 20/20". www.merriam-webster.com. Retrieved 2020-08-16.
8. "Adulthood | Introduction to Psychology". lumenlearning.com.
9. "Japan's Age of Majority Changed to 18 - Living the Japon.com". www.japan-experience.com. Retrieved 19 March 2018.
10. Weisstein, Eric W. "Vigesimal". mathworld.wolfram.com. Retrieved 2020-08-16.
11. "Definition of SCORE". www.merriam-webster.com. Retrieved 2020-08-16.
12. "Biblical Criticism", The Classical Journal 36:71:83ff (March 1827) full text
13. "CBS News", Scores of Typhoon Survivors Flown to Manila (November 2013)

External links

• [REDACTED] Media related to 20 (number) at Wikimedia Commons

• Integers
• 20 (number)