# 20 (number)

For other uses, see 20 (disambiguation).
 ← 19 20 21 →
Cardinal twenty
Ordinal 20th
(twentieth)
Factorization Script error: No such module "Factorization".
Divisors 1, 2, 4, 5, 10, 20
Roman numeral Script error: No such module "Roman".
Binary 101002
Ternary 2023
Quaternary 1104
Quinary 405
Senary 326
Octal 248
Duodecimal 1812
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.

## In mathematics

• 20 is a tetrahedral number as 1, 4, 10, 20.
• 20 is the basis for vigesimal number systems.
• 20 is the third composite number comprising the product of a squared prime and a 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 smallest primitive abundant number.
• 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.
• 20 is the number of quarter or half turns required to optimally solve a Rubik's Cube in the worst case.
• 20 is the only number with more than one digit that can be written from base 2 to base 20 using only the digits 0 to 9.

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

## In sports

File:Dartboard unlabeled.svg
A standard dartboard is laid out as 20 sectors