20 (number)

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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

An icosahedron has 20 faces

• 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.^[2]

In science

• The atomic number of calcium.
• The third magic number in physics.
• The IAU shower number for Coma Berenicids.

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^[3] (e.g. the newspaper headline "Scores of Typhoon Survivors Flown to Manila")^[4]

In sports

A standard dartboard is laid out as 20 sectors

• The jersey number 20 has been retired by several North American sports teams in honor of past playing greats:
  • In Major League Baseball:
    • The Arizona Diamondbacks, for Luis Gonzalez.
    • The Baltimore Orioles and Cincinnati Reds, both for Hall of Famer Frank Robinson.
    • The Kansas City Royals, for Frank White.
    • The Los Angeles Dodgers, for Hall of Famer Don Sutton.
    • The Philadelphia Phillies, for Hall of Famer Mike Schmidt.
    • The Pittsburgh Pirates, for Hall of Famer Pie Traynor.
    • The St. Louis Cardinals, for Hall of Famer Lou Brock.
    • The San Francisco Giants, for Hall of Famer Monte Irvin, who played for the team when it was the New York Giants.
  • In the NFL:
    • The Detroit Lions, for Hall of Famer Barry Sanders.
    • The New England Patriots, for Gino Cappelletti, who played for the team when it was known by its original name of the Boston Patriots.
    • The Philadelphia Eagles, for Brian Dawkins.
  • In the NBA:
    • The Portland Trail Blazers, for Maurice Lucas.
  • In the NHL:
    • The Los Angeles Kings, for Hall of Famer Luc Robitaille.
• The Kentucky Derby currently has a maximum field of 20 horses.
• In the NASCAR Sprint Cup Series, car #20 is currently owned by Joe Gibbs Racing. Tony Stewart won the Cup Series title twice in the #20 car with Joe Gibbs Racing. After Stewart left the team in 2009 to drive for himself in the Stewart-Haas Racing team, rookie Joey Logano took over the car until the end of the 2012 season to drive for Penske Racing. The #20 car is now being driven by Matt Kenseth, who left Roush Fenway Racing at the end of the 2012 season.
• Twenty20 is a form of limited overs cricket where each team plays only 20 overs.
• In rugby union, 20 national teams currently qualify for each edition of the Rugby World Cup.

Age 20

• The age of majority in Japan and in Japanese tradition.
• 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).

In other fields

• The term score, was used by Abraham Lincoln in his Gettysburg Address and Martin Luther King in his I Have a Dream speech.
• The letter T is the 20th letter in the ISO basic Latin alphabet.
• +20 is the code for international direct dial phone calls to Egypt.
• The designation of Interstate 20, a freeway that runs from Texas to South Carolina.
• CB slang for "a place", being short for "10–20", used in reference to a person or object's location.
• 20/20 is a late-night newsmagazine program on ABC.
• 020 is the ISO 3166-1 numeric 3 digit country code for Andorra.
• The UIC Country Code for Russia identifying member countries of the International Union of Railways (UIC).
• The letter J when encoding the serial number for intermodal (shipping) containers as defined by ISO 6346.
• In Gematriya, the system of Hebrew Numerology, the letter Kaph corresponds to the number 20.
• In Hebrew numerals, the letter Kaph(כ) corresponds to the number 20.
• The Abjad numerals are a decimal numeral system in which the 28 letters of the Arabic alphabet are assigned numerical values - in this system the letter kāf(ك) corresponds to the number 20.
• The number of questions in the game Twenty Questions.

References