# 20 (number): Wikis

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# Encyclopedia

20
Cardinal twenty
Ordinal 20th
(twentieth)
Numeral system vigesimal
Factorization $2^2 \cdot 5$
Divisors 1, 2, 4, 5, 10, 20
Roman numeral XX
Binary 101002
Octal 248
Duodecimal 1812
"Twenty" redirects here. For the village in England, see Twenty, Lincolnshire.

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]

## In mathematics

Twenty is the third composite number comprising the product of a squared prime and a prime. It is the second composite number of the form p2q; a square-prime, and also the second member of the (22)q family in this form. 20 has an aliquot sum of 22 (110% in abundance). Accordingly, 20 is the third abundant number and demonstrates an 8 member aliquot sequence; {20,22,14,10,8,7,1,0} 20 is the 4th composite number in the 7-aliquot tree. Two numbers have 20 as their aliquot sum; the discrete semiprime 34 and the squared prime 361. Only 2 other square primes are abundant 12 and 18.

• An icosahedron has 20 faces. A dodecahedron has 20 vertices.
• 20 can be written as the sum of three Fibonacci Numbers uniquely, i.e. 20 = 13 + 5 + 2.
• The product of the number of divisors and the number of proper divisors of 20 is exactly 20.
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".
• There are 20 baby teeth in the deciduous dentition.

## In sports

A standard dartboard has 20 segments

## Age 20

• Twenty is the age of majority in Japanese tradition. Someone who is exactly twenty years old is described as hatachi
• The age to be an adult in some cultures.

20 is:

## Historical years

20 A.D., 20 B.C., 1920, 2020, etc.

## 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."

# Simple English

Order ••••• ••••• ••••• ••••• twentieth

Twenty is the number that is after nineteen and before twenty-one.

The prime factors of twenty are 2, 2, and 5. (2 * 2 * 5 = 20)

Its factors are: 1, 2, 4, 5 and 10. As the sum of its factors is more than itself (ie 22), it can be referred to as an abundant number.

20 can been used as a number base. Remnants of this system remain in some European languages, for example in the English “score” (20) and the French “quatre vingts” (80, literally four groups of twenty). The old (pre-decimal) English monetary system enjoyed twenty shillings in a pound. The ancient Mayan numerical system - counting on fingers and toes - was a base 20 or "vigesimal" system.

A polyhedron of 20 faces is an icosahedron: one of the five Platonic solids. It is a convex regular polyhedron composed of twenty triangular faces, with five meeting at each of the twelve vertices. It has 30 edges and 12 vertices. Its dual polyhedron is the dodecahedron.

In Japanese tradition, adulthood is established at the age of 20. See seijin not hi (the celebration of the adulthood in Japan).

It is the number of milk teeth in a infant’s mouth.