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.^{[1]}^{[2]}
Twenty is a pronic number, as it is the product of consecutive integers, namely 4 and 5.^{[3]} It is the third composite number to be the product of a squared prime and a prime, and also the second member of the 2^{2} × q family in this form.
20 is the basis for vigesimal number systems.^{[6]}
20 is the number of parallelogram polyominoes with 5 cells.^{[7]}
20 is the number of moves (quarter or half turns) required to optimally solve a Rubik's Cube in the worst case.^{[8]}^{[9]}
20 is the length of a side of the fifth smallest right triangle that forms a primitive Pythagorean triple, (20,21,29).^{[10]} 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 decimalneon number equal to the sum of its digits when raised to the thirteenth power (20^{13} = 8192 × 10^{13}).^{[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.^{[11]}^{[12]}
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).
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"^{[18]}
As an indefinite numberedit
A 'score' is a group of 20 (often used in combination with a cardinal number, i.e. fourscore to mean 80),^{[19]} but also often used as an indefinite number^{[20]} (e.g. the newspaper headline "Scores of Typhoon Survivors Flown to Manila").^{[21]}
Formerly the age of majority in Japan and in Japanese tradition.^{[29]}
Age 20 is the age at which Levites in the time of King David were allowed "to do the work for the service of the house of the Lord", the Temple in Jerusalem (see First Chronicles Chapter 23, verses 24 and 27). In the time of Ezra and Nehemiah, following the Babylonian captivity, it was Levites from the age of 20 upwards who were assigned "to oversee the work of the house of the LORD" (Ezra Chapter 3, verse 8).
^The first such triple is the smallest Pythagorean triple, (4,3,5). These can be formed using Pell numbers that yield a Pythagorean triple of the form $\left(2P_{n}P_{n+1},P_{n+1}^{2}-P_{n}^{2},P_{n+1}^{2}+P_{n}^{2}=P_{2n+1}\right)$.
^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."
^"score | Origin and meaning of score by Online Etymology Dictionary". www.etymonline.com. Retrieved 2020-08-16.
^"Sloane's A002378: Pronic numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2020-11-30.
^"Sloane's A071395 : Primitive abundant numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
^"Sloane's A000292 : Tetrahedral numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
^Weisstein, Eric W. "Vigesimal". mathworld.wolfram.com. Retrieved 2020-08-16.
^Jonathan Fildes (August 11, 2010). "Rubik's Cube quest for speedy solution comes to an end". BBC News.
^Sloane, N. J. A. (ed.). "Sequence A103606 (Primitive Pythagorean triples in nondecreasing order of perimeter, with each triple in increasing order, and if perimeters coincide then increasing order of the even members.)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2023-07-06.
^"Definition of 20/20". www.merriam-webster.com. Retrieved 2020-08-16.
^"Definition of SCORE". www.merriam-webster.com. Retrieved 2020-08-16.
^"Biblical Criticism", The Classical Journal36:71:83ff (March 1827) full text
^"CBS News", Scores of Typhoon Survivors Flown to Manila (November 2013)
^Cleaver, Dylan (2010-11-03). Brendon McCullum: Inside Twenty20. Hachette New Zealand. ISBN 978-1-86971-238-9.
^Ziemba, William T. (2017-08-23). The Adventures Of A Modern Renaissance Academic In Investing And Gambling. World Scientific. p. 352. ISBN 978-981-314-853-6. It has always been for a long time a maximum of 20 horses ...
^Jordan, Bill. Opening Moves Made Easy: A new way to learn how to play Chess openings. Bill Jordan. there are 20 legal moves for White and 20 legal replies for Black
^Draper, Nick (2014-12-05). Exercise Physiology: For Health and Sports Performance. Routledge. p. 404. ISBN 978-1-317-90260-7. played over four 20 min quarters
^Harari, P. J.; Ominsky, Dave (2002). Basketball Made Simple: A Spectator's Guide. First Base Sports, Inc. p. 58. ISBN 978-1-884309-13-7. Duration two 20-min. halves
^"International Ice Hockey Federation - Olympic Sport". International Olympic Committee. 9 November 2020. Retrieved 22 January 2021.
^"Adulthood | Introduction to Psychology". lumenlearning.com.
^"Japan's Age of Majority Changed to 18 - Living the Japon.com". www.japan-experience.com. Retrieved 19 March 2018.
^20 - Harry Connick, Jr. | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
^Toone, Matthew (2009). Great Games!: 175 Games and Activities for Families, Groups and Children. Great Games Book. p. 145. ISBN 978-0-9798345-5-4. Much like the classic game of 20 Questions,...
External linksedit
Look up twenty in Wiktionary, the free dictionary.