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## Summary

20 (twenty; Roman numeral XX) is the natural number following 19 and preceding 21. A group of twenty units may also be referred to as a score.

 ← 19 20 21 →
Cardinaltwenty
Ordinal20th
(twentieth)
Numeral systemvigesimal
Factorization22 × 5
Divisors1, 2, 4, 5, 10, 20
Greek numeralΚ´
Roman numeralXX
Binary101002
Ternary2023
Senary326
Octal248
Duodecimal1812

## In mathematics

Twenty is a pronic number, as it is the product of consecutive integers, namely 4 and 5. It is the third composite number to be 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; a semiprime, within an aliquot sequence of four composite numbers (20, 22, 14, 10, 8) that belong to the prime 7-aliquot tree.

• 20 is the number of parallelogram polyominoes with 5 cells.
• 20 is the number of moves (quarter or half turns) required to optimally solve a Rubik's Cube in the worst case.
• 20 is the length of a side of the fifth smallest right triangle that forms a primitive Pythagorean triple, (20,21,29). This is the second Pythagorean triple $(a,b,c)$  that can be formed using Pell numbers where $a$  and $b$  are one unit apart.[a]
• 20 is the smallest non-trivial decimal neon number equal to the sum of its digits when raised to the thirteenth power (2013 = 8192 × 1013).[citation needed]

There are twenty edge-to-edge 2-uniform tilings by convex regular polygons, which are uniform tessellations of the plane containing 2 orbits of vertices.

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. There are a total of 20 regular and semiregular polyhedra, aside from the infinite family of semiregular prisms and antiprisms that exists in the third dimension: the 5 Platonic solids, and 15 Archimedean solids (including chiral forms of the snub cube and snub dodecahedron). There are also four uniform compound polyhedra that contain twenty polyhedra (UC13, UC14, UC19, UC33), which is the most any such solids can have; while another twenty uniform compounds contain five polyhedra. The compound of twenty octahedra can be obtained by orienting two pairs of compounds of ten octahedra, which can also coincide to yield a regular compound of five octahedra.

In total, there are 20 semiregular polytopes that only exist up through the 8th dimension, which include 13 Archimedean solids and 7 Gosset polytopes (without counting enantiomorphs, or semiregular prisms and antiprisms).

Bring's curve is a Riemann surface of genus four, whose fundamental polygon is a regular hyperbolic 20-sided icosagon, with an area equal to $12\pi$  by the Gauss-Bonnet theorem.

The Happy Family of sporadic groups is made up of twenty finite simple groups that are all subquotients of the friendly giant, the largest sporadic group. The largest supersingular prime factor that divides the order of the friendly giant is 71, which is the 20th indexed prime number. Both 71 and 20 represent self-convolved Fibonacci numbers, respectively the seventh and fifth members $j$  in this sequence $F_{j}^{2}$ .

## 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").

## Music

Albums
20 Related Twenty Related
Harry Connick, Jr. 20+ Baron Rojo Bob Baldwin Twenty+ All-4-One
Orbital 20% Maria Bamford Boyz II Men 20 [Twenty] FT Island
TLC Chicane
Jars of Clay Robert Cray
Africa Unite Jebediah
Kate Rusby Lynyrd Skynyrd
Hyukoh Taking Back Sunday
Pastora Soler
Sofa Surfers
Capo Plaza
John Dahlbäck
Songs
20 Related Twenty
Ebbo Kraan #20 - Fat Tuesday The Rippingtons
Seventeen #20 - Aphex Twin
Janet Jackson #20 - Tony Wilson Sextet
Gorky's Zygotic Mynci #20 - David Paul Mesler
Covenant $20 - Low (band) Car 44$20 - Qwiksand
Karma to Burn \$20 - Norman Palm
Ike Yard 20% - Furyus
Noah Stone 20% - Banfi
Alec Empire -20 - Brendon Moeller
Beans -20 - Mr. 76ix
Damián Schwartz 20 (intro) - Janet Jackson
Anarchestra 20 (Palbarrio) - El Barrio