# 20 (number)

 ← 19 20 21 →
Cardinaltwenty
Ordinal20th
(twentieth)
Numeral systemvigesimal
Factorization22× 5
Divisors1, 2, 4, 5, 10, 20
Greek numeralΚ´
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