The largest number of faces aPlatonic solid can have is twenty faces, which make up a regularicosahedron.[5] Adodecahedron, on the other hand, has twenty vertices, likewise the most a regular polyhedron can have.[6] This is because the icosahedron and dodecahedron areduals of each other.
In some countries, the number 20 is used as an index in measuringvisual acuity. 20/20 indicates normal vision at 20 feet, although it is commonly used to mean "perfect vision" in countries using theImperial 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".[7]
The traditionalage of majority inJapan, although the voting age has been reduced to 18.[9] Japanese people commemorate the twentieth birthday with personal ceremonies, and it comes with a number of legal rights like the right to marry. To represent this, the Japanese language has a special word for "20-years-old" that does not follow the rest of their numbering system. Accordingly, the word 二十歳 is read all at once as "はたち" (hatachi) rather than the expected pronunciation of the three characters as "にじゅうさい" (nijyuusai, which is literally "two," "ten," and the counter for "years old").
A 'score' is a group of twenty (often used in combination with acardinal number, e.g.fourscore to mean 80),[11] but also often used as a large,indefinite number[12] (e.g. the newspaper headline "Scores of Typhoon Survivors Flown to Manila").[13]
^John H. Conway andRichard 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, so "score" also came to mean 20."
